Hi, first of all I want to thank you for the help. I made the change you suggest on admin_groups.php and finally I have the mod working.
Anyway, I don't know why but the Invitation Messaje defined in Options doesn't appears on invitations.
I would also like to know if there is a way to allow invitaciones for registered users also (not only the new ones).
Finally, i'm sure someone else is having problems with this mod, so:
If anyone needs help with:
Broken URLS in mails
Error when clicking on invitation url
Or want the Spanish lang file
I post the solution here: http://www.punres.org/viewtopic.php?pid=9182#p9182