/etc/group, How this update starts functioning without restarting the system in Unix?
Is there any command we need to run?
Any changes to /etc/group will be made immediately.
That file is parsed when looking for access.
If you are trying to modify membership for a user already logged in though, that user may need to log out and back in for the membership changes to take effect.
If you are modifying real-person group affiliations, they need to just log out and log back in.
If you are changing group permissions for a system-like account (e.g. mysql, httpd) you'd have to restart the relevant daemons, but it is sometimes hard to know all the processes that need the new GIDs which are only applied when the process is started. Rebooting will ensure that all the proper processes are started with the new GIDs.