join in with someone—
1. meet a person to do smth. together:
- My aunt and uncle have asked me to join in with them on their holiday abroad.
2. share a cost with another person:
- We joined in with another family to buy the boat and share it for our holidays.
join up with someone—meet and form a group with a person:
- We plan to join up with the other party of climbers on the other side of the mountain.