In case of brand channel, channel admins can invite others existing/new member to to join that brand.
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> There is search bar to find which member you want to send invitation or put email id for non existing member
5> Search member and click on it or without search a member click on add/invite button
6> You will redirect to next invitation page
7> You will find a search box from where you can choose member or you can put new email id and by clicking invite all button and bellow there is a dropdown to choose assign role for invited member and then you can send invitation to one or more member for that brand.
8> Invited members will get email and notification for join request for the brand.