You can choose any background that corresponds to the theme of the photo session.
How to pose for a selfie in a group can be a real pain in the bum.
Hiring a photographer is not completely necessary: But using one can make your photos consistent and deliver the message you want to convey.
Obviously standing stiff like a soldier is not going to make the impressive.
Something like a very low stool or a stone is perfect.
All you need to do is just hold your camera upside, and allow the sky to take at least one-forth space of your selfie grid.