In the beginning, try organizing specific times where everybody will be online simultaneously. In other words, treat the website as a real-time conference, party, or networking event with a beginning and end.

Without scheduling like that, your platform will be a ghost town at first.

Also, consider seeding the group with a small number of guaranteed attendees. Ideally these people will be chosen to add value for the other guests / users. Either they'll be well known enough to act as magnets, drawing other people in to participate; or else they'll simply be interesting enough to elevate the dialogue and maximize engagement once other users appear.

Those "seed" users would need to be specially recruited. Maybe you can guarantee they'll be around without paying them anything; but even if you did have to pay a few featured guests or staff members, that may be worth doing. They solve your chicken-or-the-egg problem. They're the initial clump of snow that allows you to roll a bigger boulder and build a snow man.