Yet another option. Probably a little bit more of a hack than the accepted answer.
Enable wildcard mapping in IIS
This allows extensionless URLs to work (e.g. http://yoursite.com/Join)
- Web site -> right click -> properties
- Home directory tab -> configuration
- Mapping tab -> wildcard section -> "insert"
- Browse to aspnet_isapi.dll (see example path below) -> select file
- un-check "verify file exists" check box
- ok ok ok ok until done -> close IIS window
Create a page in DNN called "Join" and redirect it to the desired page
Add a page with the name/title Join, set it to not show up in the menu, and set it to 301 redirect to your desired URL (these are all options in the page's settings)
* typically something like C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll