"activating" changes to /etc/group

[DvB]

I decided to take the more elegant approach to getting sound working 
than making /dev/dsp world writeable so I added my user to the audio 
group. This however didn't seem to work.
I decided to worry about it later and recompiled my kernel with scsi 
support so I could use my CD-R. When I rebooted with the new image, 
sound was working  Surely I don't have to reboot in order to make these 
group additions effective :-\ ... what am I missing here?

TIA