For the new people, yes, an official fanclub exists.
No, you can’t join it. At least, not yet.
Membership has been closed for something like three years.
Signing up makes you an official member of the fanclub and usually gives you priority access to certain events or ticketing. A card proving membership will be issued to each person who joins and usually a welcome pack is sent out.
When it comes, any current members must re-apply so everyone starts on an equal footing.
At the moment, the official S♡NE membership is not open yet. But there is a process