Add class to invited users container

No issue

- Relates to https://github.com/TryGhost/Ghost-UI/commit/447c7d77ec9e532930d1f251bb42f626bebeef87#diff-acda43902fe5a16fa255b16e7b32e6d0R14
- Adds class to the invited users container, used only for spacing right now
Merge pull request #3316 from PaulAdamDavis/invited-users-class

Add class to invited users container