I get asked from time to time what the best way is to link multiple users to an appointment in Vtiger. Many CRM systems allow activities to be linked to multiple users — but vtiger does not. The short answer is there is no really good way to do it, but I will review the alternatives below.
The full form for adding/editing activities in Vtiger has an ‘Invite’ section at the very bottom where you can add in users. On Save those users get an invite email, but the invite email does not do anything. It is not an ics/ical invite where they can one click to add it to their cal. They have to manually add the appointment to their calendar. This is very confusing and not good functionally.
Another alternative is to create the appointments in your enterprise calendar system like Gmail/Google Apps or Office 360 and then sync the appointments to vtiger. This is not ideal because none of the appointments will be linked to the appropriate record in VT unless you to it manually.
The last possibility is to use the Boru Vtiger Calendar replacement. This app has ics/ical invites built in so you invite the employees as contacts, and they get an ics invite via email. That gets added to their enterprise calendar with one click and then synced to vtiger under their user calendar. Like the option above, the appointments will not be linked to the appropriate record in VT unless you to it manually.
Maybe in a future release Vtiger will change the calendar structure to allow multiple users per activity.