Hello eric and everyone,
I just saw this post and to be honest I was having the same problem and tried to find a solution and eventually I used a trick to resolve this problem,
here is what I did (And it works !),
Just use a single and common google account with the admin profile in odoo database and follow all the steps mentioned in the documentation, creating the credentials and input the client ID and secret in Odoo--> general settings--> Integrations-->Google calendar. Once it is done save the fields and go to the calendar module and click sync with google option the webpage will reroute to the google accounts and once the authentication is done, voila your odoo calendar and Google calendar will be linked together via google API, thanks to google and odoo. Once it is done go to the general settings--> Users & Companies-->Users, select the user (one by one) with whose google accounts you want to link with the Odoo calendar. Now in their users profile you will find Google calendar section among other sections like "Access rights" and "preferences" under the google calendar section you can see some information about the calendar sync and token details if those fields are empty you're good to go or else click reset account and save the page. Now switch to the employee's odoo profile and go to the calendar module, now click sync with google option, the same procedure will follow up, once the rerouting is done for authentication, and for authentication use the employee's google account and sign-in, the exact same procedure should be followed but with the employee's google account not the google account with which you created the API credentials (Client ID and Secret). Once this procedure is done now you will be redirected to the odoo database and now you can create activity using either Odoo database or the google calendar the activity will be co-dependently updated and to view the activity scheduled by your new employee's google account check the "Everybody's calendar" check box in Calendar module.
It had worked well for me ! and now if you go and check the employee's users profile in general settings you will find some new information in the field of google calendar (Do not disturb the info). And you can enjoy the feature exquisitely !
Do contact me if it doesn't work, I will be glad to help !!
