I've set up my OS X server as an open directory master first, then (experimenting), I've changed it to standalone server, then set it back as an open directory master again. Now, all the default groups I saw before are gone (Domain Administrators, Domain users etc). Do you know how to restore these groups?

Note that the groups are gone only from the Workgroup Manager UI. They do seem to be still there otherwise. id -G gives the usual list of groups.

If I create an account and makes its primary group 'staff', Workgroup Manager shows all the inherited groups properly (but not on the main list).

If I create an account and associate it to a new group I just created, then the account has no inherited groups...