We have done it, and I wouldn't recommend it (it's painful to write and maintain), but if you are feeling like pain, high level process:
create a text doc, this will be an .ics file which is the attachment to your outgoing email, clicking this attachment will allow the recipient to drop it into their cal
the spec of the ics file must be VERY exact for it to work, there is a standard - just google it - but you need to test it a million times, there are ICS generators online but be careful as some are too simple for complex situations
the hard part is the timezones, the ICS needs to take into account the TZ of the meeting location, and the users and soon your head starts to hurt.
I have a vague memory that the Aware Cal didn't have sufficient functionality, it was something to do with a situation where you need to set the TZ of the meeting location, separate to where the two or more users attending are located. You can config around it, it's kinda ok for 4 TZs, but for all TZs in the world it would be ugly.
We got it working for Australia (with 4 timezones), when it came time to internationalise it, we dropped the feature as it would be a total PITA.