With the latest update on PS4, or the controller UI on PC, in the Equipment screen, selecting an equipment slot should show you all the equipment of that type that the group has in their inventory. In co-op, you would only see equipment from the characters that you control.
There is a known issue where after asking a companion to re-join the party, the equipment filters will not work properly, until the game is saved and reloaded (actually loading a save in the homestead before asking someone to rejoin will also avoid the problem).