How to apply for a visa in Denmark for a personal business visit?

There is no requirement to have an invitation letter to receive a Schengen visa. The requirement is to present evidence that justifies the purpose and conditions of your visit. An invitation letter is only one possible way of doing that.

Since there's no absolute requirement for an invitation letter, there is also no requirement for an invitation letter to be issued by a company. If your business in Denmark is with an individual, then an invitation letter from that individual is evidence of the purpose of your visit.

Specifically, in the Schengen Visa Code, Article 14 (Supporting Documents) has this to say:

1. When applying for a uniform visa, the applicant shall present:

(a) documents indicating the purpose of the journey;

(b) documents in relation to accommodation, or proof of sufficient means to cover his accommodation;

(c) documents indicating that the applicant possesses sufficient means of subsistence both for the duration of the intended stay and for the return to his country of origin or residence, or for the transit to a third country into which he is certain to be admitted, or that he is in a position to acquire such means lawfully, in accordance with Article 5(1)(c) and (3) of the Schengen Borders Code;

(d) information enabling an assessment of the applicant’s intention to leave the territory of the Member States before the expiry of the visa applied for.

2. ...

3. A non-exhaustive list of supporting documents which the consulate may request from the applicant in order to verify the fulfilment of the conditions listed in paragraphs 1 and 2 is set out in Annex II.

For documents indicating the purpose of the journey, therefore, we may look at Annex II. Keep in mind that this is a "non-exhaustive list." That means that other documents not found on this list may also be acceptable.

ANNEX II

Non-exhaustive list of supporting documents

The supporting documents referred to in Article 14, to be submitted by visa applicants may include the following:

A. DOCUMENTATION RELATING TO THE PURPOSE OF THE JOURNEY

1. for business trips:

(a) an invitation from a firm or an authority to attend meetings, conferences or events connected with trade, industry or work;

(b) other documents which show the existence of trade relations or relations for work purposes;

(c) ...

There is the answer. A letter from your investors would help to "show the existence of trade relations or relations for work purposes."

If you have other evidence to support this, such as copies of business communications between you and your investors, or bank records of their payments, that would actually be stronger evidence than an invitation letter, because anyone can write an invitation letter.

How can they write me an invitation letter if it's them personally inviting me and not a company?

They can just write the letter.