The Easiest Speaking Game in the World
That's it really.
Draw this on the board or on a big piece of paper or click on the photograph and print that out....
The first few times you play this game, put a subject in the middle circle e.g. movies, jobs, current projects, holidays - whatever is topical at the time - elicit this from your students if you like. Divide them up into groups and then encourage your students to ask each other questions.
Once you've played a few times your students can then choose the topic within each group themselves and no group has to be discussing the same topic as the other.
You circulate and provide feedback on the lexis and structure.
I, um, think this game is straightforward enough not to include any further instructions but don't hesitate to ask questions if you have them.
update: THIS IS NOT A BRAINSTORMING EXERCISE. Encourage your students to simply ask each other questions and talk to each other: no writing :-)