Does anyone know who else is in the soul stone? And if anyone has used its power before?
What happens to Red Skull after the soul stone was taken from Vormir?
My answer is based on the movie and MCU (Marvel Cinematic Universe) only.
I believe that at the end of the movie:
Thanos is shown talking to Gamora. I don't believe he goes into the Soul Stone itself, but the Soul Stone teleports him to where the Gamora's soul is. (Purely my guess.)
As for the Soul Stone itself, at the end of the first "Captain America" movie:
Red Skull was teleported to a hitherto unknown place by the Tesseract.
We now know from "Infinity War" that:
He was actually teleported to the planet where the Soul Stone was. Based on his disembodied presence and his explanation that he is there to guide others to the stone, to attain what he could not himself, he understood how to obtain the Soul Stone, but perished while doing so.
As for how that happened, the movie doesn't really go into detail, so that's anyone's guess. Here's mine (not based on the movie):
He jumped off the cliff, but died in the attempt, which is how he understood what it took to get the Soul Stone. But being dead himself, he wasn't able to obtain the Soul Stone for himself and just drifted about as a Soul, guiding others to this Infinity Gem.
I personally thought it was a nice touch in how they introduced the Soul Stone in "Infinity War" and in none of the movies preceding it, and somehow tied it to Red Skull and the ending of "Captain America" as well.