Apparently there is a limit to simultaneous users in Google Spreadsheet. The help reports that "There's currently no limit to the number of people with whom you can share your spreadsheet," but if you try to simultaneously share with more than about 7 users, it will give you this error:

"We're sorry.

There are already too many people viewing this sheet right now. Try opening this sheet a little later.

If this problem keeps happening, please email labs+spreadsheetsgoogle.com and mention this error message and what you were trying to do when it happened."

This could be a deterrent to people wanting to share a Google Spreadsheet with many users.

Google consider 7 as too many? That's strange...
"We are going to index Googol pages!"
Good find, btw.