No, no, no. Why does every bar band PA person think they need a compressor? Compression is a studio or varsity sound thing. Not for PA's that are barely louder than the stage. And never, ever strap a compressor across the mains of a small PA. That will only make it feed back.
You probably want limiting, so you don't blow things up. But that is a different beast. I sometimes use a compressor on the bass guitar to control the level when the guy starts in with the slapping and popping. On the kick drum I sometimes use a gate to control boom or keep it from feeding back into the drum monitor (especially where a large drum monitor with sub is involved). Compression helps level things when you are blasting a large audience in an arena or something.
Think about what a compressor does. Above some threshold, it reduces the gain. In proportion to how much above the threshold the sound level is. All well and good. But below that point, it increases the gain in proportion to how much below the threshold the level is. Meaning that while the sound is loud, it is turning it down. You may as well just set the level correctly. When the sound isn't loud, it turns things up. What happens when you arbitrarilly turn up a PA system? Yep, feedback.
Compression levels and "tightens up" the sound of drums on recordings and is very popular for that reason. But live music lives on dynamics. If you squash the dynamics down with compression, you've taken all the life out of the performance. This is done in large concerts to keep things at a loud level most of the time. Producers think that keeps the excitement up. Just like on records and radio stations.
In a smaller venue, what happens is that you can still hear things off the stage, but the volume in the PA is going up and down inversely to the sound from the stage. The worst is when you strap a compressor across the main buss of the PA. Every time you hit the kick, the vocals dip down. Pumping in and out.
If you set the thing up subtly enough that this doesn't obviously happen, you may as well not bother with it. To use compressors, you have to have individual ones on each channel. And setting them so that the bleed from other things on stage doesn't cause them to pump, often requires gates. The whole thing is an art best left to folks trained and experienced in doing it. Not for a bar band PA.
If you are interested, spend some time on the ProSoundWeb reading and searching. The folks there are very helpful to well phrased inquiries. But be careful, they are pretty intolerant of folks that post newbie questions in the pro section (ask things in the Lab Lounge section) or ask questions without searching and reading the existing threads. Most questions, like applications of compression in live sound, have been discussed to death there and the threads are still around for new folks to read.