Driving home after the game last night, I had a brilliant idea. Rather than decide by DM fiat whether players who are absent get xp or not, we can use a more flexible system. Those who attended can vote whether to give a share of experience (and/or loot) to the absentee, or keep it for themselves.
hehehehehe...
To me, this seems like a more fair way of deciding it, and allows for a fresh look at the circumstances each time it happens.
Also, with regards to the... possibility for... an extra regular attendee... let me know if you know someone interested.