Send in-app invitations?

I'm building a platform where Users belong to Teams, and I want whoever creates a team to:

a) Be added as a team member, potentially with a particular status ("team_owner")

b) Be able to send invitations to existing app users to add them to the team

So far, all I've done is create the models for Users and Teams, and a has_many :through association (TeamMemberships). I started looking for gems offering invitations, but most of them seem to cater to email invitations for slightly different purposes (sending emails to invite new users to register, etc.).

Should I just roll my own, and if so, what would be the simplest way to go?

Essentially, I just want people to be able to create teams, but realized when doing that that I didn't want all users to be able to add all users to their team, or for someone to join just any team. So I'm trying to build the simplest authorization system possible to mitigate that — so if there's something simpler than invitations to do that, I'm all ears!