Thanks for your advice. I think none of the social networks got groups right. Groups is a trick thing. I think the first flaw is creating membership of groups. Instead I think people are more interested in topics(HN, reddit, twitter, etc). So I think the first step is to not have groups and instead encourage discussions around topics, links, etc.

Thats why the importance is to have a platform/features that are geared towards users.HN, reddit, discourse are great pointers btw to learn from.