This is my first time coming across this problem and i never expected it with my first time running a game.
The first group game I organised was a nightmare and it folded within about three weeks all due to unreliable players. It really dented my confidence and ego and left me wondering "how does anyone
keep a group game running?" Without me changing anything in my setup style (if anything I became a little more wary of newcomers to the site) my second group game ran very well and my third has taken off like a rocket. Player absences have begun to dent the second one a little but we had a good long run at it before it slowed down.
All you can do, as others have said, is treat everyone equally as responsible adults, and keep open communications: if your missing player has some issue keeping them from E such as ill health or failed internet access then they should
understand if you give the place to another player. If they don't, and you tried your level best to contact them, that is not your problem. At all times in this community, its about communication. I frequently communicate with my players even when the game is running smoothly, and posts are coming frequently, both in OOC threads and by PM, just to make sure everyone is happy and no-one's left out.