The 501st Legion is a Star Wars costume club, which is best described by seeing their site.
The club has members all over the world with over 2500 members and quite a few Honorary Members as well.
I am member of the Badlands Garrison here in Alberta, which is the local 501st members club, of which there are over 30 members. Want to know how many garrisons there are, check this out: http://www.501st.com/members/dsp_garrisons.cfm
If you’re interested in doing a Star Wars canon costume, check out the 501st membership guidelines here: http://www.501st.com/members/join_how.cfm
As well, I am a member of the Clone Troopers detachment; so if you plan on being a clone, make sure you join up!