How to set up a table where 2 columns together must be unique?

Hi all!

I want to create a table:

user_id: bigserial
user_name: text
user_company: text
password: text

How do I build this table so that user_name+user_company must be unique?
This way there can be only one of a given username for a given company name
but there can be more than one of a given username if they are with different
companies.

TIA,
Scott