It's said in the Unit4 photo project assignment that "Take a group portrait of three or more people" so two is not enough.
Petra, I'm not attempting to quibble with you at all, but my assignment doesn't say the same thing. I wonder if the assignments get slightly modified from time to time, so I wonder what the original poster's assignment says. Mine says,
"Shoot a portrait of a group of people. The people should be posing as a group-that is, they should have some relationship to one another (for example: a family group; a kindergarten class; a Little League team; a group of sales clerks; the winners in a beauty contest). Try to arrange the individuals in a pattern that gives the picture some visual interest. Avoid lining them up like soldiers."
Still sounds like in general it would be more than two, but it doesn't specify (or at least, mine doesn't. I'm not disputing the fact that your assignment *does* specify three or more