I wrote a blog post about 3 months ago on this topic and gave some klugey work-arounds to help vtiger users with the tough issues of linking more than one user to an activity in Vtiger.
We just did a couple customizations that addressed this issue elegantly:
- create an appt and add users via the invite section in the activity. Each user will receive an iCal appt invite that can be accepted in Outlook, Gmail and Apple to add the appt to the users calendar.
- create an appt and add users via the invite section in the activity. Our customization will create duplicate vtiger appts for each user with all the same text and linkages. This feature has been added to our calendar replacement module.
Contact us if you are interested in more details.