Right now, supporters are notables who support your clan, giving a modest boost to influence. This information is already available in the UI by hovering over a notable's picture in the top bar of a settlement. But there is no way to easily find who is supporting your clan from the other side...
Title says pretty much it :]
I would suggest so companion arrive at your highest title fief(town, castle or village), instead of random friendly city.
And show companions roles in your party in clan tab when managing members