Button link to "currentpage/"

On my site http://shuster.congressadvisors.org the big red join button is the one im talking about.

This theme is being used on multiple sister sites, so id like for the button to link to a different “join” page for each site.

so the code would be something like href=”current-site/join”

so if i was on the http://moran.congressadvisors.org page it would take me to http://moran.congressadvisors.org/join when clicked.

and if i was on http://shuster.congressadvisors.org page it would take me to http://shuster.congressadvisors.org/join when clicked.

is this even possible?