You've received great feedback so far.

Things that have worked for us:

Weekly ride - so that members don't have to wait a month for the long ride. Also builds community by seeing the group frequently.

Outgoing ride leaders - to build and maintain a welcoming community. Greet the new members, set expectations for groups, chat on route. Encourage your extroverts to step up into a ride leader role; or give them leadership tasks, such as mentoring a new person.

Start & Stop somewhere social. Try riding from a coffee shop or deli parking lot (locally own & managed, if possible) so that you have a place to eat & hang out after the ride. Again, building community, both for your club and for your local community.

Small groups - breaking down the ride into speed or intensity ensures that you can start and finish together. Smaller groups offer more opportunity for conversation. Don't underestimate the power of building group riding skills along the way... this opens up many opportunities for conversation, and is the reason you're all out there to begin with!