You can directly click on the link I gave you (the location on the Douban page is "Browse and Discover" on the navigation bar - click on the "Discover More" link next to the active group)

/group/category/1/

Then select the category you are interested in and all groups will be listed. Although it is not strictly arranged according to the number of people, basically the largest, most active group with the most people is at the front, and the farther back, the smaller the group.