I did not know that, or had forgotten. I'm sure I've seen reviews for these kind of party establishments but maybe they had the actual girls linked in the review. Seems odd that you cannot review party events in a similar vein to massage establishments as above though
Sex Parties can, and have, been reviewed, ie where, say, 3-6 named girls entertain 6-15 punters for two hours paying £50-£100 each.
Gems though is just a Swingers club where you pay an entrance fee and there may, or may not, be some ladies who may, or may not, be willing to play.
Some Swinger clubs may "host" Sex Parties which is what you may have read about.
As far as I'm aware, the OP just attended a normal members night.