Schedule date, time, and location of your walking session. Set goals personally and for the group. Keep a diary of you and the group's progress and reward yourselves along the way. Develop a buddy system. Partner with someone in the group and provide motivation and support to each other.
Variety is the spice of life.
Aside from walking incorporate other activities that you enjoy and will help you reach your goals. Encourage everyone to get a check up so they will know what their health status is. Taking care of your health is not one dimensional. You need to nourish your mind, body and spirit. It takes time and effort to lead a healthy lifestyle.