Google Apps is chargeable now and it have very small pricing for usage after completion of 30 days trail period. Google Apps for Business is having trail for 30 days.
Updated Note: Google has recently disabled addition of new domain name.
Step #1: Sign up or Login to your Google Apps
If you have previously existing Apps Account, just log-in to it or if you are new just sign up for trial version. Once you log-in or Sign-up you will be redirected to home page of Apps.
Step #2: Add domain name to Account
Click on the Domain settings tab in home page of account. Under that you can see the another tabbed page select the third tab, Domain names.
Under that tab you will be able to see the Add a domain or a domain alias button as seen in below screenshot, just click on it to add new domain name to existing account.
Google Apps add new domain name
Step #3: Add enter a domain name or domain name alias
After you have enter the Name of the domain you want to add you have to confirm site verification by various methods.
You can add two types of domain name in this account, one is the alias of existing domain name. Other one is that you can add other new domain.
Note: Before adding make sure the domain name is not already added in any other Google Apps account.
Add domain name in Google Apps
Step #4: Verify the domain site is owned by you
This is important step without this you cannot add the domain name into your Google Apps account. There are various type of site verification methods.
I always prefer to have HTML upload method because it is simple and easiest on. Once you have verified the site you should not remove the HTML file from the host.
Step #5: Add MX record to you DNS server
Once you have completed the verification you have to update your DNS server MX record and direct it towards Google Apps.
See image below to add the MX record for redirect all you domain mails into you GMail account.
Once you have complete this procedure it will take around 3 days to update your DNS record after that your will be able to receive all your GMail inbox.
You can still add more domain name in single account using same adding and verification process.
