Never donate blood in groups. Fueled by
peer pressure or
potential embarrassment, the members of your group (
co-workers,
schoolmates,
family members) may
go with the crowd when they really ought not to be donating blood.
Some people might have a very real problem telling their employer/spouse/child/etc. the reason they shouldn't donate; it's best not to put people in that position. At best, it is a waste of time (see yam's w/u in this node on the Canadian Blood Services' bar codes). At worst, it spreads disease.