The flow to join a group has generally good documentation nowadays, but if the prospective member doesn't yet have a W3C account, I believe there's a speedbump. Specifically, pages like https://www.w3.org/groups/wg/ag/instructions/, in the "If you are not the Advisory Committee Representative for your organization" section, say to "Use the Join form to notify your Advisory Committee Representative that you wish to join the group." If you don't have an account, that link dumps you on a login form. To its credit, that form includes "Don't have an account? Create one.", but for a nervous new contributor, it would be good to warn them ahead of time that they'll need to create an account.

For non-Members, the W3C Invited Expert Application link does the same thing, and could use the same warning. This one should especially point out that absolutely anyone can get a W3C account, without needing to already be an Invited Expert.