When I sing an identical song with a group, my voice tends to get louder than the group; Somehow, I think my voice has blended in with the group, but my group tells me that I am singing louder than all of them. How do I actually hear/notice when my voice gets louder than that of the group's?
Is this with or without mics? When someone sings, acoustically, they hear their own voice through the bones in their heads as well as through their ears, so they are well aware of what they are singing. Their ears, though, hear the other people singing, but that sound is only received through their ears. experience tells them when they're singing too loud, or not loud enough. That, or constantly being told where they are in the mix.
Using mics has a slightly different slant, as the mix may be pre-determined by the settings on the amp. used. That and the sound in your own head. And, maybe, the sound coming through the foldback - monitor - system, which may not actually be faithfully copying the mix on the p.a. even though it should.
The bottom line is that your voice should blend with the others, and not be louder/quieter. You have to sing in situations like this loads of times in order to discipline yourself that you really don't need your voice to be louder than the others to hear what's going on. The appropriate jigsaw puzzle piece is the same size as all the others. Just because it's important for that space doesn't make it bigger!
Listening (to the overall mix) is of paramount importance in any ensemble situation, and not enough players/singers do it. That's about it.