I try invite one guy but the error pops up: "adamos ... He/She has been already invited to a clan. Please ask him/her to clear their invitation first." I checked He has no other clan here. Why can't I invite him? Sorry if wrong board.
if he wants to join your clan, tell him reject other invites first. When he rejects, you can invite him :)
Probably he got a pending invite to accept before yours.