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.
Having selected a team you are shown the current members and given the option using the + button you can then add new members to the team. You are presented with a selection box to allow the selection of a user to add to the team.
Select the users from the pane that you would like added to the Team. Do this by highlighting the row. Once the desired users are selected, click 'Select' 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. You may need to refresh the page to see the new members of the team