I too am finding the same sort of thing with the freerolls. I recently was on a table in the early stages and 6 players with "sitting out". I think that if you check how long each Blind lasts, then work out how many games that would be and how many chips you'd put in as blinds and ante then you can decide at what time to start playing (thinking this through as I type)
Which would explain why at one point and for a fair few hands I had less chips then the invisible 6. Mainly because instead of taking it in turn playing and just over betting the big blind and taking the pot there and all their chips in quick fashion there was one "live" player who insisted in contesting every hand or so it seemed.
If I'd not played some hands I might as well have sat out.
Thinking some more...
Lets say it's 5 minute blinds. I that's roughly once round the table then you can work out how many chips you will have to play for 30/60/90 minutes. Decide when you want to rejoin...
Another thing I think I've seen is with really big numbers of players is it could be possible to hit the bubble doing this. From my very limited experience I would use the 888
10k+ player tourneys as an example. At least sitting out you aren't temped by that pair of jacks when every live player has an ace and an ace is first out on the river.
I've even seen sitting out players take pots that have doubled or trebled their stack when they were all-in with the last of their chips and a couple of live players have had a spanking when it came to show. It's something I've started watching for and checking as I have no reference to their betting patterns
Fun game isn't it.