The harp ensemble I am in is about 40 members strong. Of those 40, about 32 are beginning/intermediate harpists. The other 8 are the more advanced students. It really helps to have some of the advanced students playing the melody and the others playing the harmony and then having the beginning/intermediate students double up on it. That way, there is still something if they get lost.
Give them a signal for putting the harp on the shoulder, another one for putting the hands on the strings. Train the students how to start together by giving them a good upbeat in tempo. Train them to always keep their eye on the conductor and count. This is a great opportunity for them to get used to conducting, so be sure to use the standard conducting patterns for each meter. Choose pieces that are not too technically demanding so that they can concentrate on listening to each other and developing good ensemble. Make sure they pay attention to the musical details like phrasing and dynamics and, of course, tuning. Ask each student to play their part individually so that they understand that they can’t “hide in the weeds”. If it’s a repetitive or short piece, play around with improvisation and adding bridges in between repeats. Select at least one piece in which they must count bars before their entrances. If you have some students who have trouble with rhythm, I recommend Phil Perkin’s “A Logical Approach to Rhythmic Dictation”. This will help them enormously and contribute to fewer stops for missed entrances or haywire counting.