I enjoyed the answers on this thread as they were good suggestions!
My own suggestion to help would be to organise smaller scale group things, to help people come together who may not have done so by themselves.
An example would be, for an urban type of roleplay, creating clubs. Like in real life you can join the local arts and crafts club, make that a thing for characters.
If you have any sort of school thing going on, make the students attend classes.
The trick is to make this something that's recurrent, and not a one time event. Knowing that every tuesday your OC bob has to/can attend his HOA meeting can help new players too by giving them something to do without feeling awkward trying to introduce their character I feel!