If the subject of the sentence is the "owner" you use sin/sitt/sina. Here we have deras, so from that you can tell that it's the Swedish guys' room (or rooms).
...de "umgicks" med två svenskar på deras rum.
= They were in the Swedes' room/rooms together with the Swedes
...de "umgicks" med två svenskar på sitt rum.
= They were in their own room together with the Swedes
It is probably worth mentioning that this distinction seems to be rapidly dissolving. It is very easy to find examples of people using hans/hennes/deras when they should be using sin/sitt/sina (though, admittedly, most of those examples are probably such that there is no room for misunderstanding, for example because there is only one person involved).