There is an issue that has cropped up recently with the Trial invites.
If you are trying to send an invitation to your friend and receive the error message "You've referred the maximum number of friends available at this time. Please try again later." even if you have not used all the invitations, please ask the friend you are trying to invite to follow the steps below:
Log into http://www.ea.com with their Origin or EA Account Email address
Click on the Username in the upper right corner on the screen
Now select “Contact Preferences”
Tick the box “Contact me about EA´s products, news, events and promotions”
Once your friend has followed these steps please try to send an invitation again.