An OpenStreetMap user group is a group of people who get together to talk about OpenStreetMap, socialise and have fun. We're using the term in much the same way as an other kind of software/OS "user group", to mean OpenStreetMap enthusiasts. We don't necessarily mean "users" of the map. In fact the fun thing about this project is that most OpenStreetMap enthusiasts are better described as "creators" than "users". In any case, meeting up with like-minded OSM enthusiasts is a lot of fun!
Finding an OSM user group
There are many ways to find user groups, such as local communities or groups interested in a certain topic. Searching the OSM Wiki is a great place to start, try searching for a country, city or other sub-national place (such as a state, province, etc.) and see what you find. Often the local community will be listed on the wiki page for that place.
There are also tools to find local communities. The OSM Community Index is another great one, where local communities add their information, such as location, how they talk to each other, and how to join.
How to start an OSM user group
Just decide to do it. Then do it. The rest will follow.
Understand that you might have few, or no new attendees for your first scheduled events. Keep at it. It often takes a potential attendee hearing about you a few times before they decide to attend. You may also find that there is an early surge in attendance that tapers off until your group finds a happy size.
Select a location that is convenient for you, the organizer, and reasonable for others to attend. Often, OSM user groups meet in pubs or coffee shops. Typically, a public place with refreshments is inviting for new members to attend. Some groups, and at some times, meet at private homes. Some potential new members may feel anxious about that. Consider if you should reserve space in advance at your venue, or if spontaneous groups are welcome.
Some groups meet after work on a week night. Others meet on a weekend. Select a time that is convenient for you and reasonable for new members.
Many OSM groups meet each month. If you set a fixed monthly date some members will find that simpler to remember. Other potential members may find that it interferes with another fixed monthly meeting.
Communicating with your members
Be sure to announce your meetings on your OSM country mailing list. Especially at the beginning. Mappers might even come from neighbouring cities, states or countries to help launch your new OSM group.
Have a way to announce your next meeting(s) to members. Some OSM groups use Meetup, or GroupSpaces.com, or Upcoming.org, or facebook or similar systems to email new event details to members.
Put your local group in this wiki by adding Template:user group to your city wiki page. That will put your group on this map so folks nearby who are looking for an OSM group can find you (generated by User:UserGroupsBot)
Before your meeting
Prepare some conversational gambits
Have a few ideas of things to discuss with new attendees if conversation should slow down. The weekly summaries may provide interesting discussion points, so perhaps bookmark those and bring them with you. Some other topics of interest at OSM groups include: When and where did you hear about OSM? Where and what objects do you like to map? Which editors do you use? Do you have any questions about how to map a certain object?
Finding additional members
Reach out to other groups that might be interested in OpenStreetMap. Contact other local groups like cycling and hiking groups. Local advocacy or business groups might be interested as well.
What to do at your local OSM meeting
For the first few meetings, if you are unsure how many, if any, will attend, plan to keep yourself entertained and still approachable. Let the barista / bartender / host know that you are the "Map Group" and that others might be looking for you. Place your sign on the table so newcomers can find you. Enjoy some refreshments so that your host will enjoy having you in their establishment. Maybe read an OSM book with the cover prominently displayed until other attendees arrive. Or edit your recent mapping survey. be ready to greet new arrivals.
Once attendees arrive
Introduce yourself and the rest of the group. Offer a name tag, if you are using them. Invite the new arrival to have a refreshment, so that the host will be happy to have your new arrival as well.
Keep the conversation rolling
Depending on the groups and individuals that attend, the conversation might be completely self-maintaining. For a large group, consider offering time for people to introduce themselves if they wish to do so. Help new attendees to find an experienced mapper to coach them if they have questions about mapping and editing specific things. Ask questions of the attendees that will keep them talking to each other. You don't have to do all the talking just because you organized it.
Keep your hosts happy
Try to keep your hosts happy to have you visiting. Ask them what expectations they have for your group as far as buying refreshments, or noise level for other guests.