When shopping for an ABG, its good to remember the good advice already mentioned...they are *rarely* loud enough to be be played with a couple of acoustic guitars.
And considering you want to play it amplified w/ a group, it's also good to remember that an ABG that is loud acoustically also tends to be feedback prone when plugged in.
So there's the classic ABG dilemma!
What to do?
For your scenario, I'd recommend an ABG that was full-bodied (no cutaway) to maximize internal area and hopefully be a little louder. (note, this is just a generalization, consider the Tacoma Thunderchief is one of the loudest ABGs and has a cutaway)
Second, I'd recommend an ABG that could accept a "Feedback Buster", you know, the rubber cap that fits in the soundhole to minimize feedback. Many nice ABG's have fancy oval and other cool shaped soundholes, but then the "Feedback Buster" won't fit in those.
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Third, based on your particular unplugged needs, a small battery powered amp may be helpful in getting your bass heard over several acoustic guitars and percusion. At minimum I use a Crate Taxi at outdoor acoustic jams, and at a show or rehearsal I just use my regular bass rig (works great w/ ABGs, just need to be mindful of feedback issues).
The best battery powered amp I'm aware of and recommend 100% is the Carvin S400D - 100 watts, 10 inch driver, 4 channel, runs for hours and hours, street price around $450 USD.
Anyway, good luck with your search. ABGs are fun, but as I mentioned they have their own quirks, but then doesn't everything?
And stop by here and feel free to ask questions and look around:
Acoustic Bass fetish club
Peace