Unifying your intervention is a very important step in a successful intervention. Having a clear cut goal in mind before embarking on any activity can vastly impact the outcome, especially a drug and alcohol intervention.
The overall goal with a substance abuse intervention is long term sobriety and complete rehabilitation of the addicted person. This means getting the substance abuser to arrive at the treatment facility, complete the treatment program and return home as a productive member of society.
Talk to each person, together or separately if necessary, and convince them to work as a team, acting as a unified body with the goal of getting the person into a rehab. Send each person this list and use it as a guide as you plan the intervention.