It's not possible to tie a group to a comm site. Maybe once they allow us to make a group site looks like a comm site that will be an option, but until then it's not possible. They showed this back at ignite last year I believe, not sure where the feature went.
Hi @jackfight, MS Teams rely on Office 365 Groups to work and SPO #Comsite cannot be connected to Group, meaning you need a Group Connected Team site in the root URL to enable Teams for it. As far as I can see it won't work with a #comsite.
Just for fun I tried to use Invoke-SPOSiteSwap a normal team site with the root comsite, and when it did work the option to connect the site to a Group disappeared. ;)