Mixing live audio is arguably one of the most difficult positions in live production.
A great engineer has to balance technical understanding of the systems, artistic sensibilities, and solid interpersonal skills. This is challenging enough for professionals, so when you're working to get volunteers to fill that A1 role, there are a lot of things to think through.
Regarding the target volume level (SPL), I recommend that you work to get your volunteers to be able to tastefully judge this with their ears alone.
Before we even consider the technology behind a great worship experience, I believe there are three key things audio volunteers need to do.
First, they need to learn the art of critical listening.' This is the process of mentally deconstructing what you're hearing and analyzing the components. It's one of the keys to great audio engineering. Our ears all take in essentially the same information (notwithstanding hearing loss), but the average listener's brain will discard most of the details and just hear the overall product. An audio engineer will (hopefully) have trained their brain to pay attention to those details, and critical listening' is the auditory workout' that builds that capability. Once your brain can quickly zero in on various sonic details, it's a lot easier to figure out what's wrong in a mix and fix it, or to rapidly solve a technical problem like feedback.
It's quite easy to do some critical listening,' of your own. Simply play back an album and listen carefully. One by one, listen for details being produced by every instrument. Listen to the reverb tail on the vocals or snare drum. See if you can guess which type of microphone was used to record the kick. Listen for the room ambience from the studio in which the song was recorded. It doesn't matter if you can actually identify the kick drum mic, by the way, it's the process of focusing on those details that helps develop your ear. I recommend doing this in a controlled environment like a studio, living room, or via headphones, although you certainly can also do it over your PA.
The second thing a volunteer should do is learn what instruments ought to sound like. This can be where audio engineers who are also musicians may have an advantage. By understanding what to expect naturally from each instrument, you're more capable of building a musical' mix, and you'll be in a better position to realize when a sonic problem you're trying to tackle, is actually with a specific instrument.
Finally, volunteers need to understand what the mixing standard' is at your church. Make sure to demonstrate what a mix should sound like, and how loud it should be, and encourage them to pursue that aesthetic for the sake of worship consistency. This will also help them become better engineers, as they will then be learning how to achieve a specific target.
Regarding the target volume level (SPL), I recommend that you work to get your volunteers to be able to tastefully judge this with their ears alone. While SPL meters do have their place—and I'm certainly not suggesting you should throw yours out—I don't think anyone should just be chasing a number.
With all that said, there may be a final point to ponder, and it's going to come down to personal preference. Do you want your volunteers making mix decisions? Or, more to the point, what level of freedom should they have to tweak the mix? Should they just work with the DCA's or is it OK to adjust the electric guitar EQ, if it doesn't sound right to them?
There aren't any right answers, but it's something you should think through in advance. Be sure to communicate your expectations clearly.
Now that we've covered some non-technical considerations, let's talk equipment.
One of the most important and meaningful things you should be doing is recording and playing back multitrack band recordings for the sake of mix practice. As you probably know, it's not usually practical to have volunteers experiment with their mixing techniques with a live band. You're either trying to get through an efficient sound check, and therefore there's not much time for real experimentation, or you're asking the band to play over and over for the sake of learning.
Considering that the technology is easily within reach for many people now, multitrack playback should be a key component of any mix training regimen. While most consoles available today are capable of doing this, some make the task much easier, so make this a key consideration for any new purchase, or work to acquire what you'll need to do this with your current system.
A major audio software plug-in company offers a free piece of software that makes this process incredibly simple. In fact, we use it all the time at Lakewood Church, because of how convenient it is. Beyond that, you need a computer (nearly any somewhat modern machine will probably do) and a way to get multi-channel content to and from your console.
If your console supports Dante, it's as easy as purchasing Dante Virtual Soundcard for your computer. Some consoles also offer a multi-channel USB connection for your computer. If your console doesn't support these, you might need an option card for the console, and maybe even an audio interface for the computer. However, I strongly recommend you make this a priority, even if just to hone your own mixing skills.
If you're in the process of considering a new console purchase, you're probably thinking about ease of use for your volunteers. However, there are a couple of things to keep in mind. First, your more experienced users, and particularly the people you trust to make more significant mix decisions, can probably learn any reasonable platform. Second, you'll probably have the less experienced volunteers stick to a simpler set of operations. Given those two assumptions, it's not worth agonizing over which console is the "easiest." Of course, it makes sense to consider which workflow and feature set you prefer. It might be more reasonable, though, to consider having consistent products across various rooms or campuses, so that people need to learn fewer overall systems.
Another thing to consider in a purchase, and one of my favorite features, is "DCA Spill." As you may know, this allows you to temporarily change the console layout to show only the "members" of a DCA group on the channel faders. For example, "spilling" the drum DCA would lay out all of the drum inputs across the channel faders, temporarily replacing whatever occupied those faders before. This makes it very simple to make most mix moves via the DCA's, and then "spill" their members onto the channel faders to easily make tweaks. When you're done, the console layout goes back to normal. It makes it a breeze to find relevant faders, as they're always available via a simple and logical button press. Unfortunately, not all consoles do this well, so make sure to look into the behavior of this feature on any consoles you're considering.
Many consoles offer the ability to restrict what operations certain users can do. I think you have to weigh this option carefully. On one hand, it can keep less experienced users from accidentally causing big problems, and might even ease the mind of nervous volunteers. On the other hand, in the event of an actual issue that requires quick thinking, these restrictions can get in the way of solving problems. Therefore, I recommend giving this subject some serious consideration, but, ultimately, it's a personal preference.
I happen to lean toward the idea of not restricting the console, favoring instead thorough training, but do what's comfortable for you.
Considering that bad things can happen, please be consistent in making backups of your console programming. All consoles offer this, and there's no excuse for being unprepared for a technical failure. On that note, make sure your training program deals with overcoming technical problems. People will make mistakes, and equipment will misbehave, so make sure your volunteers understand how to troubleshoot (rather than just guessing at random button presses). Additionally, it makes sense to think through the various things that could go wrong with your system and have a plan for how to handle them. Work this into your volunteer training so they feel more confident in steering the ship.
What if you're working with an analog console? Many of the above technical considerations are moot on analog desks, but that's fine.
The most important part of working with audio volunteers is training.
Give them time to practice their mix, make sure they understand the standards and expectations, and prepare them for troubleshooting problems. If you help set them up for a win, work to eliminate the chances for failure. Above all, make sure they know you've got their back, and you'll be well on your way to technical production excellence.