Memorize: collaboration with more than 2 users broken

[godiard]

Reported by James Cameron:

Memorize activity would not properly share with more than two
participants. The symptom with more than two was that the list of
participants differed on each XO; all the icons were there, but in the
wrong order, and the "whose turn is it" highlight was inconsistent; so
one XO said it was the other XO's turn, but the other XO didn't agree.
So nobody could take a turn. We tried three times, including bringing
in each participant in turn.