1. Enable Change Data Capture for Event Relation.

2. Create an Organization-Wide-Email Address.

3. Create the below trigger.

rigger EventRelationChangeEventTrigger on EventRelationChangeEvent ( after insert ) {

    Map < Id, Set < Id > > mapEventIdAttendees = new Map < Id, Set < Id > >();
    List < Messaging.SingleEmailMessage > listMsgs = new List < Messaging.SingleEmailMessage > ();
    
    for ( EventRelationChangeEvent objERCE : trigger.new ) {  
    
        EventBus.ChangeEventHeader header = objERCE.ChangeEventHeader;
        system.debug( 'Change Type ' + header.changetype );
        
        if ( header.changetype == 'CREATE' ) {         
        
            if ( !mapEventIdAttendees.containsKey( objERCE.EventId ) )
                mapEventIdAttendees.put( objERCE.EventId, new Set < Id >() );
                
             mapEventIdAttendees.get( objERCE.EventId ).add( objERCE.RelationId );
         
         }
        
    }
    
    if ( mapEventIdAttendees.size() > 0 ) {
    
        for ( Event objEvent : [ SELECT Id, Subject, Description, StartDateTime, EndDateTime  FROM Event WHERE Id IN: mapEventIdAttendees.keySet() ] ) {
            
            for ( Id attendeeId : mapEventIdAttendees.get( objEvent.Id ) ) {
                
                Messaging.SingleEmailMessage msg = new Messaging.SingleEmailMessage();
                String strBody = '<b>Event Details:</b><br/>';
                strBody += 'Start Date and Time: ' + objEvent.StartDateTime + ' GMT <br/>';
                strBody += 'End Date and Time: ' + objEvent.EndDateTime + ' GMT <br/>';
                strBody += 'Additional Information: ' + objEvent.Description;
                msg.setSubject( objEvent.Subject );
                msg.setHTMLBody( strBody );
                msg.setTargetObjectId( attendeeId );  
                msg.setSaveAsActivity( false );
                msg.setOrgWideEmailAddressId( '0D24x000000kAqp' );
                listMsgs.add( msg );  
                
            }
        
        }
        
        if ( listMsgs.size() > 0 )
            Messaging.sendEmail( listMsgs );
        
    }

}

Sample Email: