Question about social network design

We're Wildkard - a hyperlocal network for youth sports teams. Currently, our product is team-oriented. We wish to expand this to hyperlocal communities (i.e. think 10 teams in a zip-code). Our graph structure is many:many (group-interest based) as opposed to unidirectional (i.e. Twitter) or bi-directional (Facebook).

Question we're confronting is:

• Should communities be human-created or platform-created?
• Pros & cons of each approach?
• Any mental models we can use?