In twos and threes
Definition 1 of 1
Pro Tip 1/3Small Groups
Used when people are grouped casually, not in a strict or formal arrangement. 
At the concert, fans waited in twos and threes for the band to arrive.
Pro Tip 2/3Not Alone
Implies companionship or partnership, as opposed to being solitary. 
They wandered through the fair in twos and threes, chatting and laughing.
Pro Tip 3/3Informal Setting
Suggests a relaxed environment where people can cluster naturally. 
Guests arrived in twos and threes, mingling with a drink in hand.