Last week we joined *in on* the Earth Day activities at school.
The *in on* is necessary because you are not joining the Earth Day activities, but rather, joining the others who were taking part *in* the activities.
You could also say:
Last week we took part in the Earth Day activites at school.
You can put "Last week" at either the front or the end, it's stylistic preference, but a comma is not necessary to seperate the time of the event and the event.