A strong warm-up can always help, both to get the voices warmed to and to help your group feel more relaxed and comfortable singing around each other. Don't be afraid to make it silly, people feel less shy when they get used to feeling silly around each other.
You can get creative, here, but here's a couple of ideas off the top of my head:
- sing a nonsense phrase (e.g. "plap plap plip plop plip") together to each note of a scale.
- stand in a circle. One person sings a note, "ah", then the person on their left sings that note, and so on until everyone has sung. Try to keep the volume constant, each singer fading out as the next fades in.
- get everyone to yawn and stretch (seriously), it helps to relax the throat and mouth muscles and helps to start making those longer vocal sounds. Also to fill your lungs properly. It can get quite "whacky", yawning from high notes to low notes, or opening your mouth as wide as you can. (Also, by evening, most people are really ready to yawn anyway)
As a side-bar, I am a tenor and can sing very loudly sometimes. I am wary of singing in a way which over powers the rest of the choir (Kind of the opposite of the issue you're discussing here, where everyone tries to hide behind everyone else).
Having this option, I think quite a lot about my volume in relation to the rest of the choir. The best technique I've found is to try and sing just above the level it's at (the mental image is a cork bobbing on water, at the top, but only just). This this encourages those who are a bit shy to rise to meet that level, which feels achievable, and those who are uncertain of their note are clearer about what it is (singing my part, of course).
If you all try to be that bobbing cork (not the seagul squalling above if all, or fishes hiding away beneath), you may find the group supports each other better.