Navigate to SYSTEM > PERMISSIONS > USERS
Click Add New User.
Set your desired role for the new user. Enter all required information & make sure the account is Active to allow access.
Now you'll want to flush cache and reindex if necessary:
SYSTEM > CACHE MANAGEMENT. Select Flush Cache Storage at the top right of the page.
SYSTEM > INDEX MANAGEMENT. Select All, Submit next to Reindex Data at the top right of the page.