It's like a portrait, but it's of several people, not just one. It's a still frame (as opposed to something like a video or movie), but a "still life" only depicts inanimate objects, so that term doesn't work, either.
Um... group portrait?
By the very existence of the term "group portrait", clearly such a thing exists. One can also say "individual portrait", but generally the implication of the term alone is that a single person is portrayed.