Is it that you're not loud enough or that the others are too loud? Sometimes I have to be reminded to keep the volume down when I'm playing guitar along with an unamplified mandolin. When you're used to playing with someone you get to adjust that kind of stuff, I find, but at times you need reminding.

"Several guitarists" you say. My rule of thumb is more than one of any instrument tends to be too many. Apart from fiddles. (A guitar capoed up high counts along with one played open I count as two instruments.)