AppLeo, who are you letting join the group?
It sounds like you have some sort of requirement, if you're refusing to let people join because you don't know them. You may choose who to admit, but since you've made this an announcement to everyone on the forum, it would be only fair to make it clear what your criteria for admission are.
How well do you have to know someone in order to let them join? What constitutes knowing them well enough? Does it depend on how long they've been a member here? How active? Something else?