By adding more users, we mean an option of simultaneous access to your account by several people at a time (up to 3 for Advanced and up to 5 for Agency subscription level).
Thus, all you need to do is share your login details with other users and all of them will get the same access to your account info.
Note: There is currently no way to restrict the information you wish to show to other users. For example, you only wish to show Rank Tracker information to your colleague, but not the rest (like Site Audit). This is currently not possible.