personally I would think it wise to let anyone who is really active here and well versed in hacking/programming the UBB use 6.7 beta. I mean, the more people you get involved, the more bugs can be found. Leaving people out is worse than including people who might not provide all that much help. At least there's a chance they'll help. Otherwise there is no chance at all.
Personally I don't care enough to ask if I can use the beta before it goes public. I just personally think it wise to be more inclusive than exclusive. Then again, I'm not here to tell people what to do. Just my opinion. That's all. Take it or leave it.