QGIS has a bunch of mailing lists. See Mailinglists for the different available lists.
If you are going to ask questions please read this: How to ask a QGIS question?.
Nabble (http://nabble.com) keeps a history of a lot of mailinglists.
If you go to the osgeo section of it: http://osgeo-org.1560.x6.nabble.com/ you see there is a QGIS section also, where you can do a search over all QGIS lists, or refine it to eg only the users list.
QGIS does not have a forum. Please either search in StackExchange (see below), or via http://nabble.com in the history of our Mailinglists (see above)
A lot of the development buzz will be spoken on IRC.
There is a #qgis channel on freenode.net
You can also use a web interface: http://webchat.freenode.net/?channels=#qgis
TODO: write something about the IRC ethiquette?
Depending on your country, a smaller legal entity, e.g. an association, could be established. Depending on local legislation such an entity implies that you meet at least once per year (general assembly), publish a yearly report and make the financial report available to your members. Often it also implies the creation of statutes.
Statutes of the Swiss QGIS user group as an example:
Membership application forms:
You can use these statutes and membership application forms as an idea or reference and change them to your local situation.
It is recommended to register your local country domains, e.g. qgis.ch or qgis.uk. If the domain is already taken by someone other organization, company or person please negotiate if you can transfer the owner to the local QGIS user group. The QGIS international project has registered QGIS as a trademark, which should help you during your negotations.
The website should be used to publish any material in relation to the information and management of the user group. Examples include the user meetings, case study, shared material, discussion boards, etc. Public material should be available without login requirements (e.g. do not require interested people to be a member of a social media network like Facebook or Google+). Usage of social media is recommended but should not be the only channel of information. RSS feeds should be made available at the Website of the QGIS user group.
Good luck with the organization of your local user group. Please inform the international QGIS team by registering at the QGIS community list and reporting about your progress. Please do not hesitate to ask questions regarding the establishment and maintenance of your local user group.
If you want to file bugs or issues, you need an osgeo ID. You can create an account here: https://www2.osgeo.org/cgi-bin/ldap_create_user.py
More info to be found here: http://hub.qgis.org/wiki/quantum-gis/Bugreports
All issues can be browsed here: http://hub.qgis.org/projects/quantum-gis/issues