This can be an appealing event for non-Christian friends, which they may more readily accept an invitation to than for other more formal events. It is an excellent way to introduce people to the Christian faith in a non-threatening environment.
Depending on the numbers you expect, arrange with a local pub (or coffee bar) for an area with some privacy to be set aside for your group to meet. If numbers are small then a reserved area may not be necessary. After work, for an hour, is probably the best time, although it might be possible to do this during the lunch hour.
Invite a speaker, perhaps from your local church, or maybe someone from your group, to talk about the Christian faith. A short time - maybe 10 or minutes or so is all that is required. Then allow space for guests to ask questions or discuss points arising during the talk.
Here's an example of a pub outreach evening arranged by the Camden Council CWG in London. It is a great way to talk about Christianity in an environment in which almost everyone will feel at home. It can also be used as an introduction to Alpha, or a follow-up activity after another event such as a talk or a carol service.