Hi Cori, and thanks for asking.
Please see our Sections on Creating Coalitions and on Building and Maintaining Membership for guidance on finding members. Boiled down to basics, you should identify a clear issue, mission, and purpose; then identify other people in your area with similar concerns or needs. Reach out and talk to them. Part of that agends might be to educate others about the issue or need and invite them to join you (or you decide to join them) in building mission and strategy. It can take a lot of work, but is worth the effort.
My answer to the second part of your question depends entirely upon what level of organization you want to create. If you want only a non-incorporated coalition, then you do not need any legal document. If you want to incorporate, then you may need to incorporate as a nonprofit. It depends upon the organization's purpose and whether you will seek tax-deductible contributions from supporters. Best to ask a lawyer for specific advice.