Thinking about it more, quantum theory says there might or might not be such a thing as a solo walker's group provided it was not observed. At which point, it would either cease to exist or coalesce into a group. In the latter case, the solo walkers would then cease to exist and the group would again disappear, as it had no qualifying members, to be replaced by ramblers.
In short, they can form a group provided nobody looks at them.