I ask people to let me know in end of the month reminders posts that I create, such as this one. I then check whether they actually joined as members. That's it!

Asking for more tasks increases the complexity, and I don't want people jumping through hoops just to join the community.

Membership is literally just that - joining the community. Of course, I would love participation, but I'm rewarding just getting people to join, since not every individual always wants to be an active participant.