If a player's Trident is in your group and you drop mines by it, it will have no effect on the Trident itself. Once the player docks and clicks the "control capship" button, the Trident itself leaves the group. This isn't the problem though. If your group member undocks from their Trident with your mines around it, the mines will then attack the Trident since the ship itself is not in the group but not bother the owner since the owner his or herself is in your group.

I've seen this before when helping someone farm. Their lmines will target my trident if there isn't a bot that is closer. It hasn't been enough to threaten my shields so far, but this should be fixed.