You mean how do you justify having different characters drop in and out at random times during a campaign?
If that's what you mean, I had this happen in a Deadlands game I ran.
Knowing my players availability wasn't 100%, I put them all in the same town, and had the entire campaign revolve around the town. That way, if one of them was gone, they could be 'in the next county' or something.
Also, with this kind of participation, you have to make your adventures episodic. You can still have a big sweeping plot arc, but you need to keep the adventures quick and tight.
We had this with the Heroes Unlimited mega game we had going. Since availability was an issue, we just had different heroes being assigned to different missions, based mainly on who was available. This also applied to the three GMs.
It had its fun moments, like when the GMs arranged for four teams and a couple freelancers to all run into each other during a mission in Mexico, and then sprang a major BBEG and his private army on us. Best weekend I ever wasted.
I think a site based, themed campaign is best for this. In fact, I think it's perfect. For example, characters are rogues in a Thieves' Guild for a D&D game. This type of thing would accommodate a rotating roster quite well.
The Time / Dimension Police: A setup where the PCs go through time tunnels or dimensional portals to deal with problems as they arise. Every session can be very heavily self-contained, and if it isn't, PCs can disappear and reappear in a flash of light in-character.
We played VtM, starting in Dark Ages and played to present day. however, there were multiple GMs (pretty much everyone in the group led 3 sessions each), and the sessions were in completely random order as for continuity was concerned (ie some present day adventures were played before all the dark ages stuff was done). If someone was missing from the table (or was GM for the evening) that char was simply not hanging with the others that decade.
In InSpectres, the players create a monster-hunting business. The PCs are all employees, but sometimes different monsters call for different specialties, there is more than one monster at a time, or certain employees are recovering (physically or mentally) from the latest job. The cast from one job to the next can change to reflect this, and the book actually suggests having a larger number of employees than players.