I have a question... and it's a reason why I don't support group marriages. It's purely legal.
Let's say, three people marry... then, one person wishes to divorce one party, but not the other. How would that work? Then you have issues of custody/property/maintenance. I have no particular feelings either way on the issue at all, I wouldn't want to be in one, but I don't give a toss either way... but it strikes me as a massive legal nightmare.
If you want to talk about marriage, great, but at the same time, you need to think about dissolution.
The law the Northeast passed recently requires those wishing to enter into group marriages to agree to a prenuptial agreement prior to getting married.