Mensa members have a lot on their minds, and that doesn't leave a lot of room for dealing with social niceties like whether a hug is appropriate. So when members gather, they place color coded stickers on their name tags, indicating their willingness to give and receive hugs.
Green, of course, means go ahead and hug. Yellow indicates that a hug is acceptable with permission. But anyone wearing a red dot would really rather you kept your hands to yourself.