Todd is spot on with his post.
(to the new guys: you'll find his posts are
a wealth of information)
I am a member of Clan Wallace.
The clan is really open to anyone as you can see from this quote
on our home page:

"Membership in the Clan Wallace Society is extended to the Clan and Family of Wallace (various spellings), those related to them by blood, or by marriage, and to anyone else having an interest in the purposes of the Society, which are:
Educational, with emphasis on the history of Scotland, it's public figures, such as Sir William Wallace, the National Hero of Scotland who suffered martydom for his country in 1305, it's Clans and the heritage and achievements of those of Scottish decent."

Being a member comes is handy at highland games where
you clan is participating.
The clan tent is usually a great place to fellowship with others of your clan.
A place to pull up a seat and get out of sun.
Often they will offer you food and drink as well.
Some clans may have two tents; one for the clan materials and another one for fellowship and to watch the games.