I have Google Apps setup for "domain.com". I'd like to receive mail at "firstname.lastname@example.org" how can I do this?
You can set this up as either an alias domain on your existing Apps account or as a new Apps account, entering mail.domain.com as the domain in the set up process
You will need to set up the correct DNS records against mail.domain.com.
With an Apps domain alias all users are mirrored, i.e.:
This is all done automatically for you once the alias is set up.
To set it up as an alias:
I also face the same problem, I manage a domain 'fptb.edu.ng' and wanted 'students.fptb.edu.ng' to be supported by the same Google Apps account.
I simply added the subdomain 'students.fptb.edu.ng' as a new domain (not domain alias as some people advice), I did all the verification as required by Google Apps (I used File Upload verification method).
The main issue will come in editing the MX records, Google will advice you to leave the Host as '@' which is default, but what you need to do is to put the subdomain there, so that the first 5 MX records use '@' as host which points to the primary Google Apps account and the next 5 you will add should use your subdomain as the host, in my own case, I used 'students' as the host. I am using GoDaddy for Hosting, their DNS manager sees 'students' as 'students.fptb.edu.ng', so I dont need to enter the full address, yours might be different.
At the end, I have 10 MX records.
There are many how-tos out there.
You can find one here:
That is just a really high level overview of the procedure. Google apps provides lots of help in their FAQ site.