Only admins can be assigned to a brand. If you do not see the user you are looking for below, they likely do not have admin access.
1: Click on Settings
2: Go to the "Stores" tab
3: Click on the three dots next to the brand name
4: Click on "Manage Users"
5: Click on the user's name and then click "Save"
Click here to learn how add an admin or edit a user's role.