You can simply add companies to another community you manage by clicking the "Add to another community" button. This will give the company access to another community. You can also choose multiple companies at the same time and give them access with one click
Note: Companies will be pending first in the new company, so you will need to approve them before they are visible and have access.
Choose a company or companies you want to give access to another community by clicking the check box
Click the "Add to another community" button
Choose the community you wish to add them to from the drop-down menu
Click "Add"