You can login to their TeamViewer account, either online or through the program, or they can, and add the remote computer under "Computers & Contacts". Or, what I do for users, that only need access to a system or two, and not familiar with TeamViewer, is create a desktop shortcut that all they have to do is double click it and *boom* they're remoted into their system. Simple as creating a shortcut like:
"C:\Program Files (x86)\TeamViewer\TeamViewer.exe" --id TEAMVIEWERID
More command line parameters can be found here.