As working parties are supposed to be formed to look into single problems and are normally used by committees to investigate and supply their findings members would normally be made up from those wishing to carry out the work involved. So by definition they " nominate themselves and vote for themselves by volunteering" and in practice no one would be voted for in such a case as volunteers are few and far between.
If you have a council who for whatever reason dispenses with formal committees and uses working parties outside their defined rolls (temporary investigation) then the council may look for a more formalised setting up of the working party and in practice a "who wants to do it?" is sufficient for nomination and acceptance whereas a committee needs to have a more formal setup, annual review and terms of reference and powers.( including setting up sub committees and working parties)