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.