Alycidon,
Everybody can join, as long as your airline recognizes BALPA as a union.
I work for a pan European company, am based in mainland Europe, but a BALPA member.
There are also quite a few people who are a member of the local unions. These local unions are in close contact with BALPA and everybody in my company who is a union member is represented irrespective of what union they have joined through something which in my company is called the European Pilot Group.
And yes, BALPA is the way forward, I doesn't bear thinking about what would have happened here if we wouldn't have had them!