From the dashboard, use the search functionality under accounts to find the account you want to edit.
Click on Accounts on the left hand side and click on the Account where you wanted to add the user to.
Scroll down to Users.
Click the Plus + on the right
Fill out the fields
Please be advised that If you wanted to have the user to have read only access, you will need to change the "Read-only Restriction" to Restricted. The user can access the site, but is unable to make any changes.