Every community starts as a group chat that happens to have a purpose. The interesting question is not how to get people in, it is what has to happen before they start talking to each other instead of to you.
While everybody in the room already knew each other, it is a team channel. The moment somebody joins who was not there at the start, and stays, you have something else. That transition is worth watching for, because everything you do before it is rehearsal.
Do this: Write down the date the first person outside your circle posted. That is your actual founding date, not the day you created the space.
The share of replies coming from members rather than your team. Below half, you are running a support queue with a nicer name, and you will burn out staffing it. Above half, the room can survive a quiet week from you.
Do this: Count replies by hand for one week and split them into team and member. Twenty minutes, and it is the most honest number you will get all quarter.
Tell us members, posters and who answers. We reply with the stage you are at and the one thing to change first.
If you reply to everything within ten minutes, nobody else ever gets the chance. Communities where the founder is fastest stay dependent on the founder. Leaving a question alone for a few hours feels negligent and is how members learn the room is theirs.
Do this: Pick two questions a week you deliberately do not answer first. Watch whether anybody else steps in, and who.
Most members will never post, and that is the normal shape of every community that has ever existed. They are reading. Reading is participation you cannot see, and designing to shame them into posting mostly drives them out.
Do this: Stop reporting the share who post as if it were a health metric. Report whether questions get answered instead.
This is teaching material and our own reading of standard practice, not advice for your specific community. Check anything important with your own specialist before you act on it.
How many members, how many post, who answers questions. We reply with the stage you are at and the one thing to change. No call, no pitch.