grammatical number - Are the people in 'Everyone' and 'Everybody' sentences referred to using singular or plural grammar?

I looked through some past queries and I believe this particular take on the question has not yet been addressed:

Which of the following sentences is correct?:

Everyone was working as a volunteer.
Everyone was working as volunteers.

Should the 'volunteers' be referred to as singular or plural and why?

I personally imagine 'a volunteer' as being one giant volunteer made up of multiple people and 'volunteers' as being each separate volunteer in the group.

Thanks!