You can add Customer to Company Programmatically by assignCustomer( ) function from CompanyManagementInterface.
Magento\Company\Api\CompanyManagementInterface Interface will be used for assign customer to Company in Magento 2.
You need Company id and Customer id to achieve this.
You can get assigned a company to customer programmatically by below way,
<?php class YourClass { public function __construct( \Magento\Company\Api\CompanyManagementInterface $companyRepository ) { $this->companyRepository = $companyRepository; } public function assignedCompany() { $companyId = 10; $customerId = 1; $company = null; if($companyId && $customerId) { $company = $this->companyRepository->assignCustomer($companyId,$customerId); } return $company; } }