Be/get mixed up with somebody
Definition 1 of 2
Pro Tip 1/3Negative Association
This phrase often implies a bad influence or undesirable connection. 
He got mixed up with a notorious gang, which worried his family.
Pro Tip 2/3Avoiding Judgment
Use it to describe associations without directly criticizing someone's character. 
She got mixed up with the wrong crowd, which changed her behavior.
Pro Tip 3/3Bad Outcomes
It can suggest involvement led to negative consequences. 
After getting mixed up with them, he started to neglect his studies.