Imagine you have a dungeon n dragons group.
But then a jock-sorority decided to play a trick.
They joined DnD group en-masse. Unstoppable... due to "open-membership" clause.
Then then voted to add "DnD are for nerds and forever-virgins" in the group's mission statement.
This is just a joke... but this is what "open-membership" really means.
It means you cannot associate with like-minded others, because "other-minded" people can simply ruin your right to association.