These are individual fanclubs for each SUPER★DRAGON member, supported on the app called “+KIRARI”. Joining the individual fanclubs is very straightforward, the app has English support so you should be able to figure it out quite easily! Besides credit cards, the app also accepts google pay (and iirc other local payment services as well).

In the +KIRARI fanclub, you will get access to fanclub exclusive livestreams, photos, and videos. There's a group chat with the SUPER★DRAGON members where they can talk directly to their fans (how active they are on there depends on each member of course. Tsuyoshi for example, comes to say hi every day, while others only pop up every now and then). There's also a 1 way individual chat directly with the member that essentially functions as a place you can drop messages/photos to them with a chance of them reading it. Koki for example, has mentioned that he occassionally reads these and recognized a fan's photos from these once. You can also participate in a "scratch event" which is basically a lottery where you can win various merchandise or a 1 on 1 talk with the member.