"Alle von euch werden wieder zu uns gehören!"
Thank you. That's not a usage I would have thought of, and it is a much more . . . comfortable meaning.
"All of you will be affiliated with us again" sounds better to me. To show membership of a group, I'd use "angehören" instead of simply "gehören". Der Mann gehört dem Verein an. (The man belongs to the club.) You keep the dative case for the object of "angehören", too.
Can someone explain why "belong to us again" is "...wieder zu uns gehören," but "always belong to me" is "...immer mir gehören"? i.e. why is the "zu" absent in the latter case?
Gehören is a dative verb, meaning that any object of the verb "gehören" will be in the dative case. The "zu" is not necessary, but can be used to stress to whom something belongs.
I disagree. The "zu" makes a big difference. "Zu jemandem gehören" can be used to express an affiliation. "Jemandem gehören" means possession. "Ich gehöre zu euch" = "I belong to your group", "Ich gehöre euch" = "I am your property / You can decide about me". Not quite the same thing, I'd say.