if you created the repository (or cloned a new bare repo off an existing one) with
$ git init --shared=group
or
$ git init --shared=0NNN
Git is supposed to handle permissions above and beyond what your default umask provides. At last this is true on my version of Git (1.6.3). Of course this assumes your users are in the same group.
If I needed management of users in multiple groups with varying degrees of read/write however, I'd go with gitosis. I have also heard mention of gitolite (http://github.com/sitaramc/gitolite), a gitosis fork that is suppossed to provide branch level permissions, can't say I've every used it personally though.