Unfortunately, I am not the one that actually started Medium Writers Lounge. I inherited the admin role.

The number of followers for this group is more the result of natural growth. In general, I think most groups will need some work to get the initial member base. But once they have that first 100-500 members it will be growth by association. People looking to join a group will join one with people already there with activity.

I have been admin since March 2020 and the group has grown by over 3,000 members in this time, which is entirely organic. There has been no active attempt to promote or draw members in. There is just a demand for this type of group, I suppose, and an active group with plenty of members can be a good place for people to start.