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.
We intend to introduce a hard limit of 50 members of a group in the near future so we'd recommend you stay within that limit. If you hope to attract more people than that you can create more than one group.