Remember that belonging to a clan is all about allegiance, even more so than blood ties, although those are certainly important.

Two examples from popular media:

1. Golly Mackenzie, the Ghillie at Glenbogle estate on "Monarch of the Glen" -- he is a Mackenzie, but he wears the MacDonald tartan to show his allegiance to his Chief, first Hector and then Archie.

2. Eric Stolz's character MacDonald in "Rob Roy" -- a loyal tacksmen to Liam Neeson, who also wears the same "district" tartan as the MacGregors in the film.

As several other posters on this thread have said, most clan societies will allow anyone to join who essentially "pledges alliegance" to the clan society, although some may limit them to "associate" memberships, i.e. who can vote or run for office. Other clan societies, such as the MacDonalds, are far more strict and require genealogical documentation -- which I disagree with, since the whole notion of being part of a clan went a lot deeper than genealogical ties, and you possibly eliminate a lot of good and active members that way -- Our St. Andrew's Society here in Springfield allows anyone -- Scot or not -- to be a member.

Cheers,

T.