From the Super Mario Wiki
Circling Boo Buddies are a circular formation of Boos with different facial expressions found in Super Mario World. They appear in small rooms in Ghost Houses and other haunted areas, including swamps and shipwrecks. Touching any one of the buddies can damage the Mario Bros. Unlike regular Boos, they do not become shy when looked at, suggesting Boos have more confidence in groups.
and Yellow Toad
encountering Circling Boo Buddies in New Super Mario Bros. Wii
Circling Boo Buddies also make an appearance in World 5- of New Super Mario Bros. Wii, and in various haunted areas of New Super Mario Bros. U, where they can be found with different numbers of Boos and form larger or smaller circles.
Circling Boo Buddies, as they appear in New Super Mario Bros. U
Although Circling Boo Buddies do not appear in Super Mario 3D Land and New Super Mario Bros. 2, Peepas appear instead, behaving identically. Similarly, in Super Mario Galaxy 2, there are groups of Octoboos which behave just like Circling Boo Buddies.
Another similar Boo formation is the Boo Buddy Snake.