Some forums like to setup a postcount group that elevates privileges after X posts; its possible something like this was configured and the base group was removed, its really hard to say without being one that makes changes on a forum.
Any users not in a default group (users) will need to be manually added back, and the setting for the default registration group (CP -> Membership -> Registration -> Basic Tab -> Default Group(s) for New Members) will need to be validated that users are being added to a default group.
