Two questions

1) How to make a reference to all players within a force (not the variable type force, as this is a playergroup)? I found a way round, but it's always good to ask, if there is something direct.

2) How to check which player selected which button in a dialog? I want that many players get the same dialog. Then I want to check which player clicked at which button. Is there a way?

For testing whether a dialog button has been clicked just create a new trigger with the event condition being the selection of your specific dialog button. You need to make sure that each time you create a dialog button that you assign the "last created" dialog button.
One way to work out who clicked what is just with an if/then/else statement

So imagine setting these two variables to zero starting off (for a 2-player map):
PlayerOneDialogIntVariable = 0
PlayerTwoDialogIntVariable = 0

In your trigger just have:
If (Player 1) triggered the event
then set PlayerOneDialogIntVariable to 1
else
If (Player 2) triggered the event
then set PlayerTwoDialogIntVariable to 1


Hope that makes sense (and helps).. should work.
-Nicodemus