My friend is in a different single player clan with a small base.
If my friend leaves their clan to join mine, will their building and supplies become part of my clan when they join with me? I know if there are others in their clan it would remain with the clan, but we are talking two single-player clans trying to merge into one.
From all my experiences, all will merge.
IT does ask the invited player to leave clan. All this does, is removes the clan name from everything. It is still under his ownership. When They join you, they bring the stuff with them.
That is for single player to join an existng clan only.
Private Server requires every player to be in a clan, even if there is only one person in the clan. Apparently it is easier for them to keep track of everyone.