Question about rule in DnD rules game?

If you are playing a multiplayer game (mmoropg?) that uses DnD rules.
DnD is turn based.
But its not turn based before combat is started..
So how does it start?

I mean i know it starts as soon as a enemy pops up on ur screen.. but what if you want to attack a "friend"?
Do you get a "free attack" on him before the surprise round starts or what happens?

And also how do you escape from combat in a turn based combat system?
And how do you try to chase and stop someone from escaping?
 
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