Teams
A team is a group of people you can share things with. It is reusable: its members share the roles you set, and you can grant the team access to any of your sites, each at its own per-feature roles, from the Site access tab.
Site access
Site access shows which sites the team can reach, and at what per-feature roles. It mirrors each site's own Access page: grant or edit here or there, it is the same access.
Invite by email
Invite someone who doesn't have an account yet. They get an email and join the team automatically once they register and verify that address.
Calling
A team can place one-to-one calls between its members through a media server you have bought. Turn it on in the team's settings and choose the media server instance.