The members should go to Tools->Domain Mapping where they can add or delete domains. You, the site administrator must enter the IP or IP addresses (comma deliminated) of your server on this page. The addresses are purely for documentation purposes so the user knows what they are. They do nothing special in the plugin, they're only printed for the user to see.
If mapping a domain, (like 'yourdomain.com') your users must create an A record in their DNS pointing at that IP address. They should use multiple A records if your server uses more than one IP address. If your user is mapping a hostname of a domain like www.bloggeti.net or blog.bloggeti.net it's sufficient to create a CNAME record pointing at their blog url (NOT IP address).
