If you create a brand channel you already become admin of that brand.
Now you can assign role existing member of brand as following step
In Brand profile right side corner of cover section there is 3doted(...) option.
1> Click Settings option from 3doted(...) option dropdown.
2> Go to relation tab of that brand.
3> Bellow that click on member tab.
4> In the related member tab click pen option.
5> change assign role from dropdown to admin.
6> click the tick mark to save the change.
After that member will get role assignment notification with accept decline button.
clicking accept button member directly assigned as admin role of that particular brand.