in

the verb is "to participate in"

Yes, but doesn't it depend what you are participating in?

Your participation in the meeting is required. (You must take part in the meeting.)

The new mentoring program will be introduced at this year's annual meeting. Employees with three or more years of seniority may take part in the program, and your participation at the meeting is strongly encouraged. (You are participating in the mentoring program but your participation is to be evident at the meeting.)