Users often write in that they have multiple domains that they want to use with their SmugMug site. It is often something like yourdomain.com and all of the .net, .biz, or .org variations.

It also can be for various spellings or business areas like example.com, examplephotos.com, and examplephotography.com are common.

First the bad news. SmugMug only supports one domain name at a time.

The good news is that there are ways to make this work.

Before you do anything else... Set up the primary domain using the standard instructions before attempting to work on the secondary domains.

For most domain hosts all you need to do is forward the second domain to the primary. That is typically a standard function for most domains.  Do not use domain masking as it will cause problems.  

The other methods that you can research at your domain hosts help pages are building an HTML Redirect page or an .htaccess file on your domain hosts servers.