You can add multiple Users to a Team from the Team Record.
Search for and open the relevant Team Record. Click the + icon on the Users dock at the right hand side (1).
In the Assign screen, search (2) for the Users you want to give access to the Team. Once you are satisfied with your search results, assign the results (3).
A pop up will request a Team Role for these Users. Select a Team Role (4) and click Save (5).
NOTE: After making changes to a User's access, the User will need to log out and back in to see the changes take effect.
Users can have any of four different Team Roles. Read about the different Team Roles here.