Add existing SharePoint team site to Microsoft Teams

Question

I have built a SharePoint Online team site template and a Web API in a classic team site.

Now I want to use some of the modern site features (e.g. Microsoft Teams, Office365 Groups). But I do not want to use the modern site as primary site - only the named features. When I connect to Office 365 groups (PowerShell) the modern site get the default homepage.

But when I change the permissions on my classic team site, the members do not change in the office 365 group.

Is it possible to connect the permission of modern site and exisitng team site? Maybe I do something wront during the process of connecting to Office 365 groups. Is it also possible to add existing classic team sites to Microsoft Teams?

Answer 1

Short Update:

Since some weeks it is possible to connect existing classic site collections to Office 365 Groups AND use them in Microsoft Teams.

Answer 2

Reverse your permissions management. Add the Office 365 Group to the classic site with the desired permission level, then add users to the Group. The Group is the security context in a Modern Team site and should be the same with a classic site.

You can add any site you wish as a tab along top of a Team.

With that said, you could use the process you're using now ("Groupify" an existing classic site) and just re-set the home page to your desired page.