First, read the sticky post at the top of this forum - the process of "drifting to a new guy" is one aspect of "respawning".
The solution:
respawn=group
in a text file named "description.ext" (Note: not "description.ext.txt") in your mission folder. "Your mission folder" in this context is the place where the mission.sqm is.
Also, the two following variants are also syntactically correct:
respawn="group"
respawn=4