Money, time and caring.

Money for bringing people together, giving them swag and making them feel part of a community.

Time for people to gel and start to care about each other.

Caring because if you, the community organizer (presumably) don't care, people won't care, and the community won't form.

Other things that help include an overarching mission and a way for community members to get to know each other.

Suggest you listen to https://www.communitypulse.io/ podcasts and read https://rosie.land/ articles for more.