Adding a user to a Team will allow them to see the Apps and Records that the Team has access to.
Click on the launchpad in the top left and then the COG to access the Settings area.
Select the 'Team Settings' App and then click on the Team from the list that you want to add the users to.
This will show you the current Users that are in the Team selected. From here simply click the '+' at the bottom of the list. This will pop open a pane with a list of all users on the site that are not currently in the Team that you have selected.
Select the users from the pane that you would like added to the Team. Do this by checking the box to the right of their name. Once the desired users are selected, click 'Add Selected Users' to add them to the team.
NOTE : A user will not appear in the list of available people if they are already a member of that team.