SwitchUser is a Unix-ism.
'su' is the unix command.
It's used as:
'su <username>'

(where <username> is replaced by a valid user account name).
'su' will then ask for the password, and will then make your
effective user id the <username>. If no username is specified,
'su' will change to the SuperUser ('root')

This lets each user have their own account (which they use
to log in).  You can then define an account for a role to 
be used by more than one person, which each person can 'su' into.

''Isn't this what groups are for?''

This has implications in ownership and access of files.  An
'su' account can own the files, and each user can 'su' into
that account to edit or modify the files.