@Luke Watson - Currently, only existing team members can be added to a Project. So, from your description, it sounds like you are trying to invite new users to your team, and have them added as members of a Project? If so, this is not currently possible.

Wish List

I’d recommend adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know.

If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.