HOWTOs about domains:

Setting up ASO hosting with Google For Domains
Setting up Custom Nameservers - Part 1
Setting up Custom Nameservers - Part 2

ASO accounts offer an unlimited number of domains, here is some related information:
* The Control Panel offers various options, including "Addon Domains"
* You must set the DNS for your domain, how to do this depends on who you registered your domain with and is NOT done in the Control Panel
* Nameservers: ns1.asmallorange.com and ns2.asmallorange.com
* The main domain DOCUMENT_ROOT is /home/yourusername/public_html/
* Subdomains DOCUMENT_ROOT is /home/yourusername/public_html/subdomainusername/
* Subdomains are something.yourdomain.com and are also setup in the Control Panel
* An Addon domain is treated identically as a subdomain except the domain name also points to it
* Statistics for addon domains are found within its subdomain stats. Subdomain stats are separate from the main domains in the Control Panel
* Be sure to create mail accounts in the Control Panel for all domain types, if you want mail to that domain