Note that it's a pity to have to do all this computation when admin_email is defined.... I'd fallback only if admin_email is not defined.

I don't this app at all but I have some remarks:
* On http://extensions.xwiki.org/xwiki/bin/view/Extension/Meeting+Application#HCreateanewMeeting I see that whoever created a new meeting can send an invitation email. Is that the email you're sending here? If so why send the email with the admin email instead of the user's email?
* I maintain that it would be nicer that the UI shows a red message explaining that the user (or admin)'s email is not defined (and tell him how to fill it) and grey out the ability to send the message till the user has defined his email address. Alternatively, when the user's email address is not defined, offer an input text to let the user enter it.
* Also I think it would be nice to display in the UI the email address that will be used to send the invitation before sending it.