@andrewmorris thanks for your post.
Supplier Members currently have no authority. They are simply there for information only purposes. A chance to associate multiple persons to a named supplier and to state their involvement with that supplier.
The owner of a supplier has functional permissions. If the owner chooses to make the supplier private (clicking the padlock) they can then set the permission level for other users/groups/roles for this supplier.
If the owner was to leave it should be possible for a user with the role 'Supplier Administrator' to reassign ownership. I will check this with our development team as I've just looked at this myself and I don't appear to have the option.