Once you have joined 1,000 Steam groups, you will be unable to join any more groups. This limit does not include groups that you have created or official game groups.
To join more groups, please leave as many groups as needed to be below the limit. Navigate to your groups page and click "Leave Group" next to the respective group(s).
Remember, Official Game Groups that you are following do not count towards the limit - there is no need to leave or unfollow them.
No, your steam level only affects your friends limit.