Sharing Google Calendar

Hello. I have a business partner and a couple of employees. I recently started really using our Google Calendar. The calls come in and I'm able to input the appointments in their appropriate slots in calendar "work schedule" then the trucks and partner can see the appointments from their smartphones in the field and proceed accordingly. Great!
The pitfall is that the partner would also like to input appointments  into calendar "work schedule" and the guys need to enter their time into secondary calendar "Payroll".

The problem is that I have repeatedly given all of them permission to " Make Changes To Events" but this does not allow them to add events that the rest of us can see.
I use google mail to filter my server's mail before delivering it to my Outlook so I cannot share my main gmail address with them but I did add a new gmail address and gave them the credentials. The problem with that is that it does not show up on my google calendar and there is no way to change the calendar owner.
I have sent the invitations numerous times and when the user clicks the link on their phone and is then asked if they want to add the calendar, the receive some error about invalid key or something.

What can I do to allow us all access to add events? I would really like to make Google work but alternatively, is there another calendar solution that works with desktops and smartphones?