It's to do with the Membership Policy of your website. If you have a site that does not allow users to join the site then the sign up/join button will not appear.

You can change the Membersip Policy by going to the admin settings for your site, then Users & Permissions, then Membership Policy.

Set it to Private (invitation only) if you don't want people joining or signing up for your site.

There's a bit more about Membership Policy in the documentation here.

Thanks

Joe - Voice Admin