I know that in the Winter release the "Log in to Community as User" button was added to Lightning. I did look at other pre-existing topics for this subject and none of those answers resolved the issue for me.
I am a System Admin and I have access to the button when viewing a Contact. Our core users do not, however (but they do in Classic, so I know they have permissions).
Things I checked based on other topics:
- They have "Manage External Users" permissions.
- They have Read/Edit on Accounts.
- Their Role is in the same Sharing Groups that my role is in.
- The buttons are being displayed on all of our page layouts for the Contact object.