You may, of course, join or leave the net at any point, but the most valuable observations will be those made throughout the observing period . 

To join the net, just announce your call-sign between overs. 

To leave the net, just say "73" clearly after giving your call and locator.

If you want to be a Net controller or know who is intending to run a Net then you can contact Geoff Grayer, G3NAQ.