Group (or aggregation) necessitates multiplicity constituted by properties of each individual in the group, where the priority of "I" has no privilege. On the other hand, mass (or crowd) designated, being an object of an observer, as opposite of "I" or "my" part, exists firstly with its category as the Other.
The former -- group -- focuses on the multiplicity itself, formed by many-ness, not by individual subjectivities themselves. Many-ness is to pay attention on the relation between individuals or individual subjectivities, not just to recognize its plenty.
Although identical in terms of quantity, the concept of the latter -- mass -- is founded on assumed hierarchical, qualitative difference in properties of each individual.
Are they 'mass' or 'group'?
Hundreds of climbers on the sheer Lhotse face to climb Mt. Everest, May 18th, 2012.