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.
You need to design, nurture and grow communities patiently.
Check out Rosie Sherry’s work.
For software: find users who benefit by its use and champion them. Forums, mailing lists, and recognition help in that.
Find an existing community, either with a communications challenge or which might want to benefit by use of a commnications platform.
Usenet leveraged off universities.
BBS leveraged off hobbyists.
Early online services (notably AOL) leveraged off military families on oversees deployments.
Slashdot leveraged off of the open source / Linux movement which was underserved by existing publishing platforms.
Facebook leveraged off of universities again, starting with the most selective (Harvard, Ivys, selective-admission, ...).
HN leverages off the YC community.
Story I recently ran across of a dating app was that it was used in conjunction with a few Hollywood events as a cohort-seeding formation.