Team Free Agents is for those who can fly team, but who are not on another "official" team. If you've already turned pro, you can be an honorary member.
To be a member, you need to have flown team with at least two TFA members. If you'd like the Team Free Agents button in your signature, right click on it (or two finger click for macs) and click on "copy image address." Then click on the "insert image" icon (between the link and envelope icons) and paste.