For you specific application, I would choose the Roland Cube Street EX. Assuming I was able and willing to pay for it. It's not cheap. My recommendation is based on size, weight portability, and volume capability. The Cube Street EX should work really well in an acoustic jam. But it will quickly run out of headroom for louder situations, for example if the guitars are using amps.
The EX version has two 8" woofers and claims 50W total power. I think it performs more like about 20W. It weighs 16lbs, which is about the most I would consider for this application.
The Cube Street EX can run off eight AA batteries. You could get Eneloops in a kit with a charger. A consideration, AFAIK the available power is reduced on AAs. If you need more volume you can get an optional power pack that claims to allow the amp to make almost full power. Unfortunately the power pack is fairly expensive.
One possible concern is the Cube Street EX is more of a mini PA than a bass amp. I personally like a full-range sound, but many do not. If you strongly prefer a more traditional sound, you may want to look elsewhere.
If you want more headroom, you might consider a battery powered PA. Multiple companies have introduce fairly high quality options within the last year or so. Electro Voice has the Everse 8 and 12. QSC has the CB10. These options are basically high-end blue tooth speakers. They should have considerably more headroom than the Cube Street EX and some of them are comparable cost wise. Consider the features, performance, convenience, and cost that best meets your needs.
There are tons of blue tooth speakers on the market...many of them are cheap, but also crappy. You usually get what you pay for.