Fall In

to begin to march or move in a group, often in a military context

The soldiers fell in behind their commander.

The band fell in step and began to march.

to become involved or participate in an activity

She fell in with the wrong crowd and started getting into trouble.

He fell in with the group and started playing music with them.

to agree with or support someone or something

I fell in with her opinion and voted for the proposal.

The company fell in with the new policy and started implementing it.