You can manual add and import much like customers but with a few differences.
After click on add a Pop up will appear
After you add the domain url, many of the field will be self populated thanks to Hubspot company Insights which will scrap data from the Internet about the company based on the domain and populate the fields such as company size, description, Facebook and linkedin Url and more.
This does not always work especially if the company is small or has a bad online presence, it may also imported inaccurate data so it is good to double check.
It is generaly not useful to import only Company data unlike contacts who may be contractors with no Website. It is better to import both contacts and Companies
A template for that can be seem here
The process is the same as importing contacts except you must select multiply object types such as below
And them selecting Companies and contacts
It may be useful to import Notes that can contain miscellaneous information on the contact that hubspot does not support