I am not sure if a concept of a group task is available in Trello, below is what we are tryingA number of team member create individual level tasks and push them through backlog, working, done etc. A bunch of these tasks form a group task. Let's say team member A is doing task P, Q, R and Q is an important dependency for another task S which is being done by team member B. We do not know how to link dependencies. A & B's team lead wants to know what is happening to hist group task or a mini project or whatever we call it. It might involve a series of tasks some of which has dependency relationship. Others are in a schedule and sequence. Question is how to have that group-task or mini-project view? 
Using Trello to track Progress of a task group (A group of tasks)
trello