Membership in the BFRO is by invitation only. The path to a potential invitation is by attending a BFRO expedition. Existing BFRO members need to meet candidates in person, in the field, in order to make recommendations for invitations.
Each year the BFRO hosts expeditions in the U.S. and Canada which are open to non-members. These expeditions usually last four days -- Thursday to Sunday.
You can participate if you know how to camp and you can provide for yourself completely in a camping/backpacking context.
There are a few restrictions regarding who is permitted to sign up. There is also an organizational fee ($300-$500) depending on the expedition.
See the 2013 locations and dates on the 2013 Expeditions schedule page.
On the Expeditions schedule page there is a link to the Expedition FAQ page with registration instructions and more details about these trips.