Every lib.reviews user can pass along account invitations once they’ve written their first reviews. These registration addresses look like this:
https://lib.reviews/register/<invite code>
Now, if you want to invite a friend to join and immediately also sign up to join a team you participate in, you can do so. Just pass along an invite like this:
https://lib.reviews/register/<invite code>?team=<some team>
For example, if you want to recommend they join the Vegans/Vegetarian team:
https://lib.reviews/register/<invite code?team=veg(etari)an
This will result in a sign-up page for them like this:

Sign-up page with vegetarian team preselected (Own work. License: CC-BY-SA.)
You can also offer multiple teams with ?team=<first team>&team=<second team>&team=<third team>, and so on. It’s a bit of an Easter Egg feature for now; in future, I hope to integrate it more with the team management tooling.
This is the first new feature for lib.reviews since the recent modernization work. It seemed only appropriate to start with a feature that was open as a pull request since (checks notes) 2018! Thanks to Patrick Noll for the original work on the idea.