Lack of physical activity is a major problem contributing to diseases and poor health. Nowadays, mobile fitness apps serve in important roles in encouraging and facilitating people to do more physical exercise. Many apps focus primarily on individual behavioral strategies, such as displaying individual steps to encourage physical activity. Such strategies help evoke one's internal motivation such as peer recognition and competition achievement. However, such apps usually de-emphasize or ignore interpersonal behavioral strategies, such as team rank. And group-based strategies are very important in aspects such as peer recognition and can facilitate more physical activity. This research explores the design strategies of group-based dynamic approaches for encouraging physical activity in small-size groups. The development effort takes into account the different roles of mobile devices and laptops and the evaluation explored the effectiveness of the design.