In 2020, I created a dating group for South Africans to promote a dating offer. I started by adding friends with SA profiles and automatically inviting them to the group. I also posted sexy SA women photos in the group with interactive questions like "single lady here, would you date me?", that would give me some likes and comments which is engagement. I also shared this posts on South African dating groups.
I kept doing that for two months and after some time, fb algo started promoting my group, I started getting like 500 members per day.
Just start posting content, share with groups and also invite friends. After a while, the algo will pick up your group promote it on related groups