I am not sure if this would be a new topic to include BUT people also are constantly trying to register their own team mates and without a unique email are unable. A team captain should have a way to add mates to their team without having them go through the whole process each time!

Bump.

This would be particularly helpful for DIY campaigns in which a user starts solo but their effort gains traction and other people want to join their effort.

Back in the retained engineering days, we tried to accomplish this with the creation of the current feature where people can join or disband the team (if captain) from within the participant center. There was a myriad of reasons why these were the only two actions that could be accomplished by the participant automatically post-registration. But, I wanted to mention that we did customize our participant center to utilize surveys so the participant would at least see the "form a new team" option within the team management options within the participant center. With the survey, it's seamless to the user but collects the information necessary so that the recipient of the survey submission (an admin) would have what they needed to form a new team with the requesting participant as the captain. 

I've provided a screenshot of our implementation but wanted to let you know this is at least an option if you wanted to discuss with your Services team.

This is a great idea. I can see following up with participants not on a team with an email encouraging them to get their friends to join and instructions on how to create a team.