In addition to what Rochey said there are three alien races.
http://half-life.wikia.com/wiki/Xen#Ecosystem
The Xen creatures are actually on the run from the Universal Combine. They lost their war as well and in the events of Half Life 1 they were the first to arrive at Earth. Apparently they've been taken over by the Combine as Headcrabs and Vortigaunts are used in the Combine. At the end of Half Life 1 Xen was controlled by Gman's employers, who we never really find out who they are. The game shows dead US soldiers so apparently the US government helped but no one really knows.
http://half-life.wikia.com/wiki/Race_X
This is the race seen exclussively in Opposing Forces. While they could be the Combine proper chances are they are not because the amount of teraforming creatures that break into the Black Mesa compound in the original Half life. Unlike Xen's purples and pinks these seem to be greens and yellows. Even less is known about them.
http://half-life.wikia.com/wiki/Combine
This is who we'll be facing on Earth. They seem to be mining Earth's natural resources. Mainly the ocean levels are dropping.
Now as far as each race's transporters go the Combine seem to have the most primitive. They can only move from one dimension to the other and require massive power generators. Dark Matter fusion generators which might not even be possible in Star Trek's universe. Humans have figured out a better way to do it at Black Mesa which slingshots across the boarder world (Xen) to the location. Xen seem to have perfected this and can transport in the same realm. Whereas Race X seems to take it to the next step and is able to do it without any technology.
Because when dealing with multiple universes things that are impossible with one universe might be possible with another. Warp for example might not be possible at our stop after Half Life (10 cookies if anyone can guess whats a major sci-fi universe we have left completely untouched[ edit- isn't Star Wars]). Which is why a Xen technology drive is so much better than the one we're using because it operates on a slightly different set of laws of physics. Reason we're having every law of physics slightly different is hell we have mages, mutants, people who use warp, people who use psykers. In reality a lot of those would be impossible. Why would a gene in someone's body allow them to control ice? By allowing different universes to operate within the laws established in-universe then we resolve any unrealistic problems.
Far as beaming the Vortigaunt out we don't know where he is. Or even what he looks like. We just know he's one of many in the city.