I run a writers' group. We meet fortnightly, which is enough of a gap to get some writing done, but not so long that if you have to miss a couple of meetings, it hardly seems worth returning.
At each meeting, we set an assignment topic, such as, 'An Emergency,' or 'Hidden in the Shadows.' It's optional, and can be responded to with a poem, short story, non-fiction comments or playscript. Those who are working on novels might try to include the phrase or situation in their next chapter, or if the book's going well or it's not appropriate to the subject, they might choose not to do the assignment.
We start with a round-up: we go around the table talking about what writing we've done since last time, problems which prevented writing and events which helped our progress.
Then we do a writing exercise: usually starting with a list of words and phrases describing a topic such as a house, or an activity, such as getting your hair cut. We discuss the lists and topic, then have about 15 minutes to write a short piece, which doesn't have to be finished.
We have a 15-minute break with tea/coffee and a chance to go to the loo/have a ciggie/chat about other subjects. This time has been invaluable as we have become a friendly social group, not just a group of writers.
After the break, we read our work (assignments or extracts from other writing) and give feedback.
At the end, we confirm the date of the next meeting and set a new assignment. I also post this on our Facebook page.