Q: Volunteers とはどういう意味ですか?
A: Freiwillige


Q: Volunteers report a sense of satisfaction at enriching their social network in the service of others.

in this sentence, what does 'in the service of' mean?
A: “In the service of others” roughly means “helping others” or “serving others” in this case. The sentence talks about volunteers, so this makes sense.
Q: Volunteers have made the provision of food for the sufferers of the disaster. この表現は自然ですか?
A: Volunteers have supported the provision of food for the victims of the disaster.

I regularly volunteer here with a big charity in the uk and I also teach their induction programme. I changed made to support because unless the volunteer made the food themselves then this would be factually incorrect.

I also changed the word sufferers to victims just because that is more natural in the UK.
Q: Volunteers group この表現は自然ですか?
A: "Volunteer group"

Leave the "s" out.

If you were talking about the people that are volunteering, you can say volunteerS to pluralize the many.

But since you are using the word group after volunteer, it already suggests more than 1 person is a volunteer.

If you have more than 1 group of volunteers, you would then say:
Volunteer groupS. Because now there are many groups of volunteers.
Q: Volunteers are ordinary people who do extraordinary actions この表現は自然ですか?
A: Instead of "extraordinary actions," say "extraordinary things."