If you want to assign branches to a power user, you can do so by clicking on the icon under the Branches column on the Manage Power Users page. You will be redirected to the branch page. You can choose the branches which need to be managed by the power user.
Press SAVE button to assign power user to the selected branches. You can also unassign branches from power user using the same page. The power user will be able to manage (as per their profile) the users for the assigned branches only.