Staff members may be added by the account owner from the 'Users' tab on the home page. Under that tab select 'new user' then enter their details. (Once they log in they may edit their password themselves if they wish from their own 'Users' link.)
Once set them up at appropriate level of usage (partner, manager, senior, junior or peer reviewer) they may be assigned to jobs by editing 'Users' from the side-bar once within the job.
The type of user determines their access to certain features. Only a partner can perform final sign-off over a job. Only Partners and Managers can create review notes. Seniors may edit pages but juniors cannot. Juniors may only see the jobs they are specifically assigned to. Peer reviewers also only see jobs they are assigned to like juniors but may also add review notes.
See also Setting Up Users