I am searching for two opposing terms:
How to call a person who is employed by a company?
How to call a person who is not employed directly be a company, but cooperates with this company, for example, a freelancer?
Would these terms "internal worker" and "external worker" be appropriate?
How to call a person who is employed by a company?
Answer: You can call them employees of the organization
(noun) a person working for another person or a business firm for pay.
[Dictionary.com]
How to call a person who is not employed directly be a company, but cooperates with this company, for example, a freelancer?
Answer: You can refer to them as third party. However, I think the term freelancer conveys the intended meaning very well.
(adj.) Relating to a person or group besides the two primarily involved in a situation
[ODO]
In the IT industry, we often use the term third party vendors or third party auditors (besides internal and external) to refer to personnel not directly employed by our company but use their services when appropriate.
The general official term , in USA and Canada is
Independent contractors
from the point of view of the person who pays for their service:
People such as doctors, dentists, veterinarians, lawyers, accountants, contractors, subcontractors, public stenographers, or auctioneers who are in an independent trade, business, or profession in which they offer their services to the general public are generally independent contractors.
and
Self-employed
from the point of view of the person paid.
If you are an independent contractor, you are self-employed.
Whether these people are independent contractors or employees depends on the facts in each case. The general rule is that an individual is an independent contractor if the payer has the right to control or direct only the result of the work and not what will be done and how it will be done.
IRS: https://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Independent-Contractor-Defined
Canada: http://sbinfocanada.about.com/od/taxinfo/a/contractor1.htm
Then depending of the field, you could use a more precise word, such as sub-contractor (building), consultant (I.T), guest (academy: guest speaker, guest lecturer) etc.
