I understand that Peter Quill was able to handle infinity stone without destroying himself possibly because his father is a (powerful?) alien of an unknown species. But how were the other Guardians of the Galaxy able to handle the power of the stone?
Over the course of the movie, we see three different "entities" try to use the power of the stone:
- The Collector's assistant (daughter, I think?) tries and quite spectacularly fails, because she's just a mortal and can't handle it.
- Ronan tries, and briefly succeeds, before welding it onto his war hammer. Note that, in the "flashback" scenes during The Collector's exposition, we also see the stone being wielded through another object, which appears to make the stone much more manageable. We know that Ronan is not a 'normal' mortal, so he was able to survive the stone for the short time it took to attach it to his weapon. However, it was clearly trying to destroy him even during that time.
- The entire Guardians team, who were the only ones that seemed able to control the stone. But note that Peter's ability to hold the stone was hinted at in the final scene as possibly being due to his non-human father. The rest of the Guardians were only in contact with the stone through Peter.
Basically, the way I interpreted the scene is:
Quill was acting as a buffer, taking the brunt of the force of the stone for himself, but able to handle it because of his nature. The rest of the Guardians were acting as a kind of "focus": by supporting him as a team, it allowed Peter to channel the energy of the stone directly without overwhelming him.
(On a side note: this is also the reason why the Infinity Gauntlet exists in the first place: apparently, even an immortal being like Thanos would be unable to control the stones directly; he needs to embed them into an object that he can use to control and channel their power in order to use them.)
Quill was able to handle the power because he is only half human. The other half is Spartoi, a humanoid species with much longer life spans than humans, (upwards of 200years). Between this and the fact that he was sharing the power of the stone with the other 3 all of whom are non-human allowed the power to be controllable for a short time.