We created World Walking with individuals and small groups in mind. It has not been designed for large groups or to support large walking challenges, particularly ones they are 'time-limited'. If you are aiming high, we recommend you check out Big Team Challenge which has been specifically designed for what you are looking for.
You can comfortably have 50 or so people in your World Walking group. We recommend you keep your group size to below 100 members.
You can, however, create more than one group.