Sometimes people just decide they don't feel like a "full person" anymore, like they've lost a part of themselves, and wish to blend in with another person.
Sometimes it's 1 person going into 1 person, sometimes it's 5 going into 1. It's a very mutual thing where they "blend" by agreeing with a person/s that they want to join their body.
Later they may decide to split again because they feel like they're ok and are back to their normal selves so they gain their body back.
I hope this makes sense, I'm so tired after work all day.