Sorry to bump.
*I found this thread when searching Google for adding a signup link in Discourse’s global banner.
What I found to work best is to use an invite link (take care with link expiration and # of invites limit). It will look something like this:
forum.domain.io/invites/xXxXxXXXx
When a non-member clicks, they are taken to the signup form with a “invited by yourname”.
When a member clicks, it ignores/bypasses the invitation signup form and takes members straight to the forum homepage. (unless you have set up a custom redirect for the invite to a specific topic)
The other options have some caveats:
takes/shows even existing members to the signup popup.
shows already logged in members the login popup which does not work
I’m guessing invites may not have been in place in 2016. 
Hope this helps.