The integration of LinkedIn for Office 365 will not take place directly in the Office 365 Admin Center, and instead in the connected Azure Active Directory. An enable feature for user groups is currently not available, and the either/or principle will apply.
Users proceed as follows to enable LinkedIn integration:
Login Office 365 –> “Administration” -> “Admin Center” -> “Azure AD” (opens in a new browser window) -> “Azure Active Directory Admin Center” -> “Azure Active Directory” > “User Settings” – and then check the box to enable LinkedIn.
The following video shows how to enable LinkedIn in the Azure Active Directory Portal. The relevant passage is at minute 02:04.