Since the --shared option just sets the permissions on everything in the repository to group-writable you could do this manually later:
$ chmod -R g+w the/repo/path
Plus, add
sharedrepository = 1
under the [core] section in .git/config. Shared repos also have the following receive option defined by default (which you may or may not want):
[receive]
denyNonFastforwards = true
Note: In order to decide whether you want denyNonFastforwards: This option means a merge never happens in the shared repository, which, in turn, means there is never a merge conflict on the shared repository. Instead the push is rejected forcing the user to do the merge in their local repository where it’s much easier to fix and where it doesn’t interfere with other people’s use of the shared repo.