I realize that this thread is two years old; however, if you found this via search because your "Share to Teams" is either not showing or the one you added to your Outlook Ribbon is greyed out, let me share what solved that for me.
I found the solution in this document: https://support.microsoft.com/en-us/office/connected-experiences-in-office-8d2c04f7-6428-4e6e-ac58-5.... Here is the relevant section from that document:
Controlling these experiences
Office includes these connected experiences. If you'd like to turn these experiences off, go to any Microsoft 365 application - such as Word, Excel, or PowerPoint - and go to File > Account > Manage Settings (In Outlook it's under Office Account). There you can disable or enable, either category (or both).
The fix, however, is turning the experiences ON in this case. After I checked the box to "Turn on optional connected experiences," and restarted Outlook, the Teams button appeared, and the one I had added to the Ribbon now has color instead of gray.
If it helps, I found that document in a link from this one: https://docs.microsoft.com/en-us/microsoftteams/teams-outlook-share-teams.
