Hello,
This can certainly be done, but it's highly recommended to have some experience creating code for EPESI. Some documentation that can be of help:
http://www.epesi.org/index.php?title=EPESI_Tutorial
Basically, a new field would have to be added to the Companies (Requests) and you can give access (or even a button on the ActionBar) allowing another company employee to add his/her company to the requests. Then, you can use display callback to show all the request companies with action buttons (accept/decline) and perform an action respective to the choice made. Of course the actions would only be available for people from the Company we're talking about.
All this can be fairly easily done with a custom module and no modifications are necessary in the EPESI code.
Kind regards,
Arek
