In Bathory, I think it was just Jonas and Quorthon as the main members at the start. Session (I don't know if that's the right word for this - I do know about session musicians, but I don't know if they were or not) musicians or part timers would have been the other members. Just think of Nine Inch Nails - that's just Trent really, but others play on the albums.
Quorthon's stuff released as the band/person Quorthon was just himself though.
That's what I can gather from it all, but it is a bit of a mystery.