A team lets other people get the alerts from your monitors. Creating one comes with the Power User plan. Joining a team you've been invited to works on any plan.
A team on Power User has room for 3 people besides you. An invitation nobody has answered yet takes up one of those places.
Whoever made a monitor is the only person who can pause, edit or delete it, even after it has been shared.
Open the link in the email and press Accept Invitation. You need to be signed in with the address the invitation was sent to. The team's monitors then show on your Monitors page under the Team filter, and you get their alerts.
To stop a team's alerts without leaving it, press the bell at the top of the team's page and switch off Receive notifications.