Brink is designed to balance out ranks and teamplay. It'd not be fair for a 20 rank to beat up on Level 1 players unless in the chosen mode, Old School. Play Old School.
If the level 1 players CHOOSE to be targets, and the level 20 players CHOOSE to allow them into their game, then they should be allowed to go get beat up.
At present, there's no option to allow lower-level players to join your games.
EDIT: There should be "my rank only," for players within the same bracket as you, "my rank and higher," for added challenge, and "any rank," which allows players both higher AND lower rank to join your game.