[MrFlippant]

Hello Greg

I would like to join in but it will be half way through.

B.

Well, don't let that stop you.
Even during the recording, it's very casual. You can leave early, or join late, or both. People do it all the time. Come when you want to, and leave when you want to. Please don't ever stay away just because you can't be there at the start and until the finish. 
If anything, I'd say there's no need to make a fuss about either one by saying "sorry I'm late" or "I have to go now, bye!" ;-)