Collaborative editing is generally the hardest part of authoring an I-D as you will need to keep on top of multiple changes from multiple sources.
GitHub is widely used by IETF WGs for tracking issues and the related changes to Internet-Drafts. The following tools/services have been created by community members to support the use of GitHub:
internet-draft-template and i-d-template are a pair of tools that work together to provide an automated Markdown I-D build system in GitHub. These are described further in Drafting in Markdown.
The ietf-github-services organisation contains a number of repositories providing supporting functionality, including:
iddiff is a web service that provides a side-by-side comparison of two documents. It is generally used to show the differences between two version of the same I-D to see what has changed, or between a submitted I-D and a working draft.