Two solutions:
1/
You can add an account with the menu : Admin / Administrators.
Create a new one.
Select the module you need to allowing for this user.
2/
Or with the menu Admin / User Management.
Add a new user if it doesn’t exist.
If the user has already an extension, click on it and then, go to the Tab FreePBX Administration GUI.
Alow him to login on the GUI.
Select what you want here for this account: Administration Access
Select a landing page.
That should be enough to do what you want.
The second solution will be better if you use any ways to get accounts (LDAP…Etc).
The first works too, but I think it will be deprecated soon.
