As far as I understand, the party member or clan member is different from the last game. The clan members are served to be a supporting cast for the clan. All the settlements (castle or city) are given to the clan and the clan leader. If the clan leader (either other clan or your clan) will have all the income from those settlements. You can use that income to support a party to have its own troops.
So there is no point for a field to be given to the party and it can not be done in-game. Instead, you can distribute the income wisely and choose to have your clan member (party member) to have their own troops.