Click your avatar in the top right corner of the screen and select 'Add Teammates' from the list shown.
Click 'Invite staff member' as shown below.
Fill in a user ID, e-mail address, first name and last name then click 'Send invite' to finalise the invitation.
Note: The 'User ID' is a unique handle used to identify the user. Tudodesk recommends a nickname or other association but all user ID's must be unique.