Every plot has a leader - the one who started it - and it is only he who can invite others. There is however nothing stopping you from being a member of one plot and start another one as leader with the same target.
Hmm so lets say I have a borther who invited me to a 'usurp the clan leader title' plot (lets say our family is from the clan leader dynasty), then I make my own plot to do the same.
If my brother launches his plot first and theres a civil war, will my plot be lost?
Or can I launch my own plot and make a 3 way civil war?
Or my plot target will change to the winner of the civil war?