A Letter of Invitation is required from the company or partners to be visited in the Democratic Republic of the Congo. 
The  letter must be on company letterhead and signed by a representative of  the company, and should include the applicant's name and the name,  address, dates of planned visit, description of purpose of journey to  DRC and telephone number for a contact person at the overseas company. 
This  letter must be notarized at the City Hall in the DRC and legalized by  the Direction Generale des Migrations (DMG) the declaration of liability  or "engagement de prise en charge" issued by the Direction Generale des  Migrations (DGM) is now required to apply for a visa, except if you are  being invited to DRC by a government office in which case an invite  from that office will be accepted. 
A fax or email copy is accepted by the Embassy.