Thank you in advance.
What does "offsite events" mean?
Events that are not held in the main building of the company/group/school/other establishment.
I would suggest that these are events that are not happening on the main site - the main place of activity.
What about events that are not held or happening at the primary edifice of the corporation/conclave/educational facility/alternative organisation, or not on the principal terrain/the principal location of happenings.