For multiple people collaborating on the same document there is multiple free options. One is with version control like GitHub where you can invite poeple to commit pieces of the document through pull requests or directly to the main branch. The other more dynamic approach is JSFiddle with it's collaboration feature, where you can work on the same content at the same time. Or the in-between, have your main index file in JSFiddle and include the seperate sections from remote git repository so you keep the version control.