Log in to the admin interface of your site.
Then simply go to the user icon in the admin bar on the left to access the Users admin area
Then add a user with the Add User button at the top
Once they have been added you will see them in the list of users in the table and next to their information you will see the details button to view their details.
Open their details and scroll down to the "Group Membership On All Sites" property.
Assign them to the "My Staff" and "TopSale Site Admin" groups.
The user will now show up under the "My Staff" tab in the user admin interface and they will now have access to administer any section of the site.
If you desire for them to have access to only certain areas, instead of giving them TopSale Site Admin access, only assign them to the groups pertaining to areas you wish them to have access to. Such as Form Admin to only let them manage forms and form submissions such as a contact us form.