Hello,
is it possible to assign all persons from a group of persons to a photo?
No. Use the Link Person command and tick / select all the persons you want to link to an image.
I will add that this function has a great feature that I stumbled upon.
The order in which you tick the names is the annotation order that iMatch will apply. Makes it an easy single step to both assign people and have the names in the order you want.
This was one of those nice surprises in working with iMatch - there are many.
Michael