Replace in:
mopidy-party/mopidy_party/static/controller.js

mopidy.tracklist
	.index()
	.then(function(index){
	returnmopidy.tracklist.add({uris: [track.uri]});

with:

mopidy.tracklist
	.index()
	.then(function(index){
	return mopidy.tracklist.add({at_position: 1, uris: [track.uri]});