(Bob did not read this entire post.)
I think you could apply these ideas to any type of ensemble...
Tone quality
Intonation
Expression and style
Interpretation
Rhythm
Articulation
Dynamics
Stage presence
"Other"
Just do your best. If someone asks you, they must know something about you. If they don't, then they find out the hard way right?

If you're judging many groups, take those categories (or they might even give you a sheet for this), give each category a set of number 1-5 or whatever you want, and use that to compare ensembles to each other. It's a rubric. If you want, before you start, write out what a 1 really means and what a 5 really means. That can help. After several groups, it's easy to go brain dead.
They should give you a score to follow. If not, then just do your best.
At least, the judge sheet rubric gives you some kind of numerical rating. You could weight the categories if you want.
It depends if they just want comments or if they want you to rank the groups and decide who's the best.
If you're doing a lot of groups, you really need something like a rubric so you can be a little more objective. The first and last groups should have the same standard of comparison.
The groups might appreciate your comments. It's always good to give a positive comment first, then some advice, then another positive comment. A positive "sandwich" around the real criticism.
If they don't ask for numbers, don't give them numbers. That can invite people to nitpick and argue in an attempt to get more points. If you tell them who you thoguht was best and maybe why, you sound intelligent and people may agree and not argue. If you tell them the first group got a 12.4 and the second got a 12.5, then someone will come up ready to argue. If all they want is comments, then great, give comments. Some groups don't care for "fluff" and positives. They only want the negatives so they can focus on fixing things.
I read more... I see it's a pop band/Christian group.
If they sing, you can rate them on
vocal tone
enunciation -- can you understand the words?
pronunciation -- do they sing "ur"s?
Of course, ask the organization what your supposed to do, what they expect, how you're supposed to judge. They may answer all this for you.
Hope that helps some.