How do I tag a company or user on the social feed?

To tag a company or user on the social feed, all you need to do is launch  "Share a Post" 

Type "@" followed by the company or user you want to tag. A drop down will also appear where you can select who you want to tag in your post.

Next, you have the option to set your audience.

Public: Viewable by all

Connections: Viewable by only your connections

Community: Viewable by all members of the domain

Click "Share" to share your post. The company's users will receive a notification that you've tagged them in a post.