Following requests on another forum for a group challenge feature this has been implemented. Anyone can create a group, and these groups can then be joined by other members. Please note that each person can only create one group. Filling in the group name field and clicking "Go" will merely rename the group if the creator has previously created one. Members can also only belong to one group. To delete a group you will need to have all members leave the group and join another (or join the ungrouped members group).
Please note: You can only create a group if you have made at lease one purchase.