Event and Attendee Sync Behavior for Users Not Assigned to a Sync Configuration in Lightning Sync

The assumption is that no application syncs data with your Salesforce.

When an event or a meeting invite with attendee(s) is created in the Salesforce Calendar by one user that has included other users as attendees, this event will be saved on the creator calendar as well as on the calendar for all its attendees to help them track the meeting invite. This is an out-of-the-box Salesforce feature.

Also if an event is created by a non-syncing user, Lightning Sync will not sync the event, even if the attendee is assigned to a Lightning sync configuration and a syncing user.

Now if you are syncing data between Salesforce and Outlook or Gmail, using Lightning Sync or Einstein Activity Capture, the same applies. Please review the below scenarios for more details and examples

Examples and explanation of this behavior  

Lightning Sync = Enabled and Connected
Assume there are 2 Salesforce users, where:
User A = Assigned to sync configuration and synchronizes events bi-directionally
User B = is NOT assigned to a sync configuration

Scenario 1

a) User A creates an event in Salesforce and adds User B as an attendee. Both organizer and attendee see their own version of the event on their Salesforce calendar. For User A, Lightning sync will sync that event with their Outlook/Gmail Calendar. An invitation is sent to all the attendees (User B) from the organizers’ (User A) Microsoft or Google accounts.

b) If User A creates an event in Outlook/Gmail and add User B as an attendee, since the event Organizer is a syncing user, the event syncs to their Salesforce Calendar along with the attendee information (to the attendee field on an Event). Both organizer and attendee see their own version of the event on their Salesforce calendar.

Note: When an event with attendees gets created in Salesforce, child event records are created (in Salesforce) and assigned to each attendee who is a user. A child event record isn't created for the attendees who are contacts or leads.

Scenario 2

a) User B, a user that is NOT assigned to any Lightning Sync configuration and is NOT syncing events, creates an event in Salesforce and invites User A (assuming the attendee is set up to sync Salesforce to Exchange, Salesforce to Google, or both ways.). Each attendee sees their own version of the event on their Salesforce calendar. The syncing attendees’ (User A) version of the event doesn’t sync.

b) User B, creates an event in Outlook/Gmail and add User A as an attendee. The event for User B will not sync to Salesforce, however the event for User A will sync from their Outllook/Gmail Calendar to Salesforce (assuming the attendee is set up to sync Exchange to Salesforce, Google to Salesforce, or both ways.) On the version of the event synced to Salesforce, the attendee (UserB) is the organizer and only participant. Other attendees aren’t visible.

See Also:
Event Attendee Sync
How Events Sync with Einstein Activity Capture