There are many ways to form small groups for learning activities. One tool that I like for its flexibility is to give each participant a playing card, then form groups on the basis of poker hands (or some other game): Four of a kind, Flush, Straight, Straight Flush, etc.
You can have face cards (or any other set) be observers or note-takers. You can have jokers be facilitators. You can form groups of any size.
You can randomly distribute roles or you can use this to form groups on a more intentional basis -- for example groups of people with a similar job, or mixed groups -- by handing out cards to people with shared characteristics (all restaurant workers in the group get 3s or Hearts).
The mix of suits and numbers, combined with the variety of "hands", allows you to easily form and re-form groups of various sizes and compositions.
You can also use this technique to decide the order in which groups report back (highest hand first).