To open the invite form, click on your avatar on the very far right of the header bar and select Invite Others from the resulting drop down menu.
Enter the emails of the people you wish to invite, select a role for them (Employee, Contractor, or Vendor), and add them to an existing team. You can add them to more than one team by holding down the Ctrl key when you click the teams.
If you enter a third individual, another row will appear. This will happen for each additional person you add, so that you can invite as many people as you wish. Once you are done entering your desired individuals, just click Invite Now!