One of the things you could try is planning a lunch or dinner or trek or some other activity that is enjoyable and sort-of informal-ish. Invite 1-2 of your founder friends and ask them to bring 1-2 of their friends along. Make sure the group doesn't become so large that members subdivide into subgroups. This way not only you, but your other founder friends can expand their network. And, if on the off chance that someone doesn't get along well with someone else, they still have someone in the group to enjoy the fun activity with.