Family and friends can play an important role in motivating individuals with drug and alcohol problems to get into treatment and stay in treatment. In order to do this the family members must educate themselves about the nature of addiction, they need to learn what they have control over and what they do not and be willing to give up control of the outcome for the addicted person.
The addicted person needs information about their condition. If family members aren't clear about the nature of addiction it will be hard to give clear information to their loved one. The addicted person will spend a lot of energy rejecting this information so if the family member understands that this is normal it will reduce their anxiety and help them to continue to give information to their loved one anyway. When you give information to an addicted person it must be simple and straight, such as, I love you, you need help, here is a list of treatment programs. Addicted people do not need judgments, threats or attempts at manipulating them into treatment.
One of the more difficult skills that family members and friends need to learn is how to allow the addicted person they love to experience the consequences of their addiction without trying to save them. Addicted people need pain. Without pain they will not be motivated to seek help. One important note, it is not the job of the family to create pain for the addict in hope that this will make them go into treatment. This is very important. I have talked to many parents that kicked their adult child out of the house thinking this will make them go to treatment, and the addicted person does not go. There are no simple solutions here. If you choose to kick someone out of the house it should only be done to make your life better, to protect your environment. If this results in the addicted person going to treatment great, if it does not, that's OK also because that wasn't the goal. This would be a natural consequence for the addicted person as a result of their drug use and they will have to deal with it as opposed to a manipulative consequence created by the parents to create pain.
In order to play the strongest role in this with an addicted person in your family you must get education, get support, pass on the information to the addict whether they like it or not, learn to give up trying to control the outcome for the addicted person, allow the addicted person to experience the natural consequences of their addiction, and when not sure what to do seek out the support you have been developing for advice and more support.