I'm sure the Historians here will agree that the Clan system originally had little to do with "blood relations" and was more centered around sworn or implied loyalty to a chieftain.
By those rules, anyone can join any clan as long as you hold some feeling to that clan.
Steve has summed it up quite nicely, so I won't repeat it.

Some clan societies don't understand the history of the clans and become rather elitist in their membership requirements. They are only hurting themselves.

Cheers,

Todd