Subscription only for gmail accounts?

I have recently created a new domain using googles apps. Since my
personal gmail account is rapidly filling up, I decided to move all
my mailing lists off of my personal gmail account onto a new email
address of my new domain using google apps. When I tried to subscribe
to this group using the google groups webpages, it did not give me the
option to subscribe from anything but an actual "" account.
I don’t have the google groups option with my domain of google apps.

I tried subscribing my new email using…

[email protected]

This works, but I am unable to post messages from my domain. I got an
error message…“You do not have permission to post to group
rubyonrails-talk. You may need to join the group before being allowed
to post, or this group may not be open to posting.”

Can I only use my gmail account to subscribe to this list?