fall in with

fall in with  {v.},  {informal}

1. To meet by accident.

Mary fell in with some of her friends downtown.

2. To agree to help with; support.

I fell in with Jack's plan to play a trick on his father.

3. To become associated with a group detrimental to the newcomer.

John fell in with a wild bunch; small wonder he flunked all of his courses.

Compare: PLAY ALONG.