I wouldn't know exactly how it works, but it's obvious to see if you ask around. Statistically it should be impossible for two people who know eachother to meet when a potentially infinite number of worlds are being pulled from, but there are numerous groups of people here who seem to know eachother. That's not counting all the shinki who could potentially know others here as well.

There are... also methods for travelling between worlds willingly. But a great deal of energy would be needed for that, in addition to knowing where you want to go.