Shortly after signup, you should receive a confirmation e-mail containing an activation link. Click it to activate your account. Request another one if you did not receive the e-mail within a few minutes.
A quick search can yield similar bug and avoid opening duplicates. Take a minute or two to search our issue tracker or our forum for recent bug reports that could match yours, but don't worry if you open a duplicate: a contributor will handle it :)
The Framasoft GitLab instance is hosted at framagit.org. You have to sign-up here, using your e-mail account or by way of a third-party token provider.