killpg() sends the signal sig to the process group pgrp. See signal(7) for a list of signals.
If pgrp is 0, killpg() sends the signal to the calling process's process group. (POSIX says: if pgrp is less than or equal to 1, the behavior is undefined.)
For the permissions required to send a signal to another process, see kill(2).
There are various differences between the permission checking in BSD-type systems and System V-type systems. See the POSIX rationale for kill(). A difference not mentioned by POSIX concerns the return value EPERM: BSD documents that no signal is sent and EPERM returned when the permission check failed for at least one target process, while POSIX documents EPERM only when the permission check failed for all target processes.