I think it's probably too late to say 'before this gets totally out of hand...' but nevertheless...
Point 1 is that the issue around full club membership mainly pertains when a club has significant assets such as a hut. If there are uninsured members of that club and there were a claim, then that club may find that despite the fact that the majority of its members are insured, the club assets still need to be sold to pay the claim for those members who are not insured. This is why ALL members of a club must join the scheme in order for the club's assets to be reliably protected.
(Point 1a is that all club members benefit from the insurance, so BCA believes that all club members should pay towards it, but that's not actually relevant to the discussion at hand.)
Point 2 is that the scheme will always protect properly insured members of a club for their individual liability no matter what the status of other members of the trip is. However, a claim against an uninsured guest will not be paid by BCA and so unless the guest has other insurance which provides equivalent cover, the other members of the party may not be able to make a meaningful claim against the guest.
Point 3 is that the best way to be sure that cover is as watertight as possible is for all members of the trip to be insured by BCA, but it's not an 'all or nothing' situation.
Point 4 is if you have questions about the BCA insurance, the best thing is to contact the BCA, not seek answers from some bloke on an internet forum...
Regards to all
Nick.