Jump in / out during play

First, a brief observation. From the way you describe the game here, my gut is telling me I ought to caution you against two things:
- lots of player down-time
- lack of player interaction

I don't know ANYTHING about your prototype though. There was a recent podcast describing pointers for designing co-op games, so I suggest you look into that for a seasoned perspective.

Beyond that, I'd like to see the term "pick-up" play to be used to describe the situation you describe. Someone new shows up late to the backyard pick-up ball game, they jump in on whatever team needs a player. Players head out whenever they need to go, and if necessary team members are shuffled in an attempt to keep them "even."

I hadn't read your other threads, and it seems like there are more details elsewhere. But I gathered my impression from your description here, specifically:
- the game is (up to) four hours long
- the fact that the game allows players to jump in and out at will, without interrupting the flow of the game

When I see that, I often wonder what the game is about, and how you go about making it happen. Does it need to be that long? Can the game continue even without input from other players?

I also qualified this at the outset by stating that I know nothing about your game. So if you feel like you've addressed those concerns, then great! Pay my impression no mind. I'm completely on the outside and I think it's reasonable that I don't have a clue about your design. :)

I will be using Drop In / Drop Out Term. Similar to co-op / multiplayer video games.

Stormy

Your welcome Frank.

Bows respectively.

Stormy

This is similar to what might happen with XP in an RPG where the gamemaster expects some players to be unavailable some of the time. Back when I had time to run such a game, I'd set up what I called a "drag rate" which set the minimum XP for any character in the party. A new player joining the group (it happens) would get to build a character with XP at this "drag rate." For established players, a character was still present even if the player was absent, though the character received no XP. This would ensure that any Plot Coupons would be available when needed. If someone dropped out for enough sessions in a row to get caught up by the "drag rate" (which didn't happen), the explanation was that character had been learning something from all that relatively passive participation. I figured that "rewarding" absence would be less disruptive than crippling the party with an underpowered member.

The tricky part of this is what to do with the playerless character during encounters. An established group of RPG players will gloss over the insignificant presence of the character during that time, but things may not be so simple in a more directly goal-driven game. Should the character sit out (and cause encounters to scale down) or continue on autopilot?

If the enemies have a similar scope of abilities as the characters, an "AI" might already exist in the game to use as a respectable autopilot. Sadly, the AI might outperform some players.