Similar to team instances you can't enter a battle in a higher bracket than you've unlocked, so the level 70 can't enter the 80+ Jin rescue

However you can enter a battle lower bracket than you, so a level 80 can enter the level 70-79 Jin Rescue

Unlike a lot of events this ones difficulty is based on the leader of the team, not the highest level.

So if the level 70 is leading the team you can enter together. If a level 80 is leading though the level 70 won't be able to enter.