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.
Best Answer
With Winter '18, Users can manage Community Users in Lightning Experience. Snippet from the release notes:
In the issue mentioned here, Users had different page layout that as of Admin, and Admins were able to view the button in LEX. For any button to be visible in LEX, it needs to be available in Salesforce Mobile and Lightning Experience Action section of the page layout. Adding it in there will make any buttons available in page layouts in LEX (refer screenshot below)