yeah man then i would definately invest in a PA system. If you guys don't have a lot of cash you can get one used or just rent one. With 2 guitars, vocal, and bass thats only 4 inputs which is the size of a standard small PA system. The drummer should be loud enough by himself. Use your amps (again, facing toward the audience to increase volume if need be) and the PA. Heck, the PA would probably pump enough volume but you wouldn't be able to hear yourselves.
If you don't wanna spend money, then just use the amps you have. In the case of the vocals you can get a 1/4" cord splitter at radio shack and put the mic in one of the amps (probably the loudest one since it does cut the volume in half). Or if one of the amps has another input use that. If you need to get another amp for the vocals you could take this approach:
1. Lots of money - You should've just bought the PA
2. Not a whole lot of money - Buy a cheap (any brand) 25 - 50 watt amp and plug the mic into that. Vocals are on a higher frequency so they carry more than the bass and so forth.
3. Absolutely no money at all - Ask one of your buddies from school or someone you know to just borrow their amp for the day. Treat it right and you shouldn't have any problems.
For the mic, i don't know what you use, but for around $100 you can buy a really awesome vocal mic, Shure SM58. They are REALLY good mics for the money. Shure makes great products. If you don't have money, just use whatever you got. ALso, on the EQ for the mic i would cut the bass some and boost the treble.
The volume of the jams will depend. When the people get there, they will absorb the sound (their bodies) so it won't be as loud as it was when you practiced (im guessing you'll practice before they come). So, VERY IMPORTANT, go up to whoever is in charge (probably your uncle) and just be like "hey after we play a couple songs or so could you tell us if we need to turn the volume up or down, or any suggestions on the EQ or anything". They should tell you how loud they want it.
Other than that man, if you have other questions id be glad to help, i hope this helps, i know its a lot.
mark