If you are looking for the commonly used contemporary terminology, it seems to me that your inquiry has been answered. So I feel you may be searching for a new phrase that is a nuanced descriptive of a "lie-in" or "die-in" (as suggested in your title, "describing a group of people...")
One could riff on the old by creating something new:
Vie, as in:
Battle, as in:
to try to overcome; struggle against or contend with
The demonstrators are vying to overcome some perceived injustice, vying in a struggle for a voice, vying as an act of contention against powers whose eyes and ears are closed to their needs or opinions or rights. They are vying with the words affixed to their very bodies, with their bodies affixed to the very ground, as an expression of the power that a dead body holds ubiquitously, as a spector which must be seen, acknowledged, respected, and responded to. So that their voices may be seen, acknowledged, respected and responded to.