An agile coach I worked with recently lead us through an excellent process relating to this.

- as a group, explore the problem space (ask more and more questions, diverge your thinking, embrace the waffle)

- as a group, pick something to focus on (converge)

- individually sketch out a solution or two (diverge again)

- individually, asses each others' sketches, identifying what you like and dislike

- synthesise a solution from all the sketches (we did it by voting on the detail points in the previous step; converge)

- storyboard, as a group, with people taking it in turns to drive

- build

- test on users (diverge)

- review (converge)

The coach told us this was mainly derived from Jake Knapp's "Sprint" book.