I haven't seen the research, but it is an interesting question.
My preference:
Small groups (2 to 4 people), ride to the right on narrow, busy roads. In some cases one can squeeze two wide into a shoulder or bike path if both wish.
On not very busy streets, ride side-by-side until a car approaches, then have the outside rider either drop back or pull forward into single file.
There seem to be anecdotal reports of road rage. And, I think car drivers and cyclists work better together if they all think they are sharing the road and respecting each other's needs. There was a
Recent post of "road rage" after a car couldn't get around cyclists quick enough
Once one gets to larger groups (5+ riders?), then:
- Spread out a bit if the situation allows (road ride).
- Form a draft line if one wishes.
- Or, in some cases, especially in urban areas, it may make sense to group up a bit so the group can pass through stop signs and traffic lights as a group.
I've been in urban groups that enveloped cars, and that doesn't work well, unless it is a huge slow moving group (100+).