Interesting information about taking passengers - does the CFI approval part apply only to club gliders, or does it also apply to privately owned gliders?
The general rule at gliding clubs is that you can't launch (club glider or your own glider) without the approval of the CFI, usually delegated to the instructor of the day. Thus what the rule is, and what gliders it applies to, depends on the CFI.
At my previous club the rule was that only instructors could fly with non-solo pilots, both in club aircraft and in their own aircraft. Some clubs will differentiate, so that who flies with you in your own two-seater (if current etc.) is up to you. Others will allows anyone who meets the BGA minima and the CFI's approval of their flying skills to take non-pilots in club gliders.
The problem the CFI has in allowing non-instructors to do this is in deciding which of the club pilots are capable of taking passengers safely, bearing in mind that the responsibility for flying safety at the club falls primarily on the CFI. The esaiest route is to limit this to instructors because they've all achieved an externally-assessed standard. Avoiding internal conflicts is important at a gliding club - if some members spend their time complaining about unfairness, there is a real danger of the club taking sides and falling apart!