If you're already within a clan on the same server, you may not be a member of more than one clan in each of the games. This rule is adaptable and in certain games, if there are good reasons, it may be possible to belong to more than one clan. These issues are handled on an individual basis, please ask the SWAT-Administrators for further information.
Else if you're in a clan on another server, but on this you're a freelancer, noone will stop you joining one, just remember that making more accounts to be on more clans on the SAME server is against the rules of both server and clans, as noone wants spies.