The age-old problem...
Simply put (without getting into a physics discussion), guitar is loud in immediate proximity to the stage and bass is loud about 20-30 feet away from the stage. Guitar is unidirectional, meaning wherever you point the speaker cab is where the sound will go - like a beam - and bass is omni-directional.
So, how can a group of musicians deal with these discrepancies without resorting to a brawl?
I play both bass and guitar and here's a few simple ideas that have worked for my projects:
1. The guitarist(s) should tilt / angle their speaker cabs up at them (like a monitor). They'll be amazed at how loud they really are (and how much treble they are using) when that cab is blasting them in their face. If they can place the cab in front of them and angle it up, it's even better, but that usually isn't practical.
2. The guitarist(s) should use less overdrive and less reverb / echo and find a tone that cuts (usually high mid-range). This will be a tricky sell because most guitarists crave that "bedroom tone" dripping with saturated OD and lots of delay. What they don't realize is that tone sounds great by itself, but gets murdered in a group setting, so they instinctively reach for the volume knob.
3. Run everything through the PA and use stage monitors for stage sound. This one may be impractical for an open mic / jam situation, but if there's a house backline, just throw some mics on it and inform all players that they are to use the house amps and keep stage volume to a minimum because the PA is doing the work.
4. If the stage is hollow, the bassist needs to adjust the bass volume to compensate because the hollow stage will act as a primitive sub-woofer. This also affects the bass drum, but that doesn't have a volume knob...
5. If there isn't a house backline, limit the size and power of the amps that will be allowed on stage. Inform all players ahead of time that stage real estate is limited and use of smaller equipment will be required if one wishes to play at the event.
Good luck!