Sometimes you just want to get busy and DO something with a group of people. No problem! Often you can do a lot of excellent work without any formal structure. After you've been working together for a while and have a sense of how you want to address your issue, what your priorities are, and so on, it might be a good idea to sit down together and come up with your group's:
The goals might be broken down into short-term and long-term goals. This gives you a framework for reference, and helps you evaluate ideas against a common agreed-upon set of statements.
Informal groups can be very responsive and certainly all big issues were first tackled by people working together informally. Informal groups can be the best ones to deal with urgent local problems, where quick action is the top priority and people are focussed on clear and obvious goals.
Informal groups sometimes run into problems when their volunteer and personal resources get stretched too thin. Then a group might want to solicit outside funding to help them keep the work going. Some funders don't mind working with an informal group, but most like to deal with organizations that are registered or incorporated under provincial or federal law. This is mostly due to liability and accountability issues.