Troop tunnel gives extra damage increase every level, but you can only get this by starting a rally.
It doesn’t make a difference whether you or your teammate starts it. The important thing is that the person and people with the “maximum level building” provide more damage bonus.
You can view the bonus as “building” in the attributes page. You and all your teammates in rallies can use the bonus. Those who start the rally and participate will receive the bonus.
If you have any other questions don’t hesitate to ask. Have a nice day.