I would ask our affiliated translators, but I need it -right now- and I'm not sure I can reach them fast enough. Any help would be greatly appreciated -- thank you very much in advance!
ETA: thanks to akibare and naatz who helped me clarify the request:
"Membership" as in: "static membership in a club" or "The fact of belonging to some group" (as a single, non-declined word, not as part of a sentence)
I apologize for not thinking of this right away!
ETA2: all solved, thank you all for your great help!