Allow your teammates who wouldn't normally have visibility to a company record the ability to view and edit any company information on the company profile page. Shared companies are accessible via the Companies tab and Search bar.

To share a company with another user, visit the company profile. Click Actions, then click Share.

A couple of reminders: 

1) Sharing companies allows the user to see the company record, but the associated people and deals will follow the original visibility privileges unless those records are also shared with the teammates.

2) Any user that can view the company record can also share the record.