'Trials' are different from 'Dungeons'. A 'Trial' is a single battle instance. Trials do not allow you to add members. If people leave (in Ifrit HM) it's usually because of 1 (or more) of these 3 'core' reasons:
A) Party is not fully competent (ie: Players have FATE Ground to 50 and haven't learned their jobs or rotations).
B) Party is Undergeared (Titan HM Requires 3200+ Health minimum during Phase 5).
C) Party is not learning from their mistakes.