Aside from your main Online Store Manager login that you are given upon sign up, you can add an unlimited number of additional user accounts to your store's interface. This section can be accessed by going to Settings =>General =>Administrator Access.
By creating additional users, the store will allow you to have a separate login for each member of your staff that needs a login to the store's administration pages. Through the use of permissions on each Administrator login you create, you can limit which areas specific staff can or cannot access in your store. Furthermore, the store will also keep a log of changes made via the Online Store manager, so you can see who may have made changes to your store as they are made.
More information on adding new users can be found here.
Note: As with the main administrator login that you are given at the time of sign up, additional administrator logins will also require a PCI compliant password change every 45-days.