A simple workaround would be adding a Sign Up button to the Welcome message:
<a class="Button Button--primary" type="button" title="Sign Up" href="./signup"><span class="Button-label">Sign Up</span></a>
Since this requires Flagrow's Direct Links, which isn't yet updated for beta 8 (even thought it works for me) and reloads the whole page, it would be better to open the signup Modal with js, but I can't quite get it to work. It has to be something like this: app.modal.show(new SignUpModal()), but no idea how to access the SignUpModal here.
EDIT: Ok I solved the problem, with the whole page being reloaded. You still need Flagrow Direct Links and it isn't pretty but here it is:
<button class="Button Button--primary" type="button" title="Sign Up" onclick="app.modal.show(new app.routes.flagrowDirectLinksSignup.component.component.prototype.createModal)"><span class="Button-label">Sign Up</span></button>