How Do You Get Past The Team Rocket Grunt In Radio Tower In Pokemon Soul Silver?

To get past him, you must go to the basement. In there, you find a
Rocket Grunt recruiting people. He says for you to join, so you put on
a costume. Then you can go to the Radio Tower. The other Rocket Grunt
lets you past until your rival rips your costume off and shows your
identity. But you still get to past.