Suggestion to replace a Facebook group?
- A group open by invitation/approval
- A group open for anyone

I would like it to be a tool that is
- free and open source
- encrypted
- easy for an ordinary man to join and engage in from an ordinary computer, tablet or mobile
- in other aspects, compare to a Fb group

I'm thinking of Matrix, but it is not that easy to join; not so easy to get an account.

Do you have any other suggestion?