If I queue with 4 friends (making us a full 5 people team), do we get matchmade (?) with another 5 people team, or is it possible to fight 5 people that do not know each other?
After playing over 1000 games, my and my friends experience is: You can be matched with and against any formation of players.
But when playing with a Party of 5 against 5 Individual Players, these Individuals may be of a higher skill rating then you, to compensate your improved communication. This is a common practice in Matchmaking Algorithms and all data seems to support Dota 2 works the same way.