If you already use Diadoc then it is quite easy to know what contractors are connected to Diadoc: contractors with whom you have interchanged documents are already in your list. To add a new contractor is easy — you shall just send it a request to add the contractor in the list. Such a request can be sent only to those using Diadoc.
If you are not a Diadoc user yet you can receive information on whether your suppliers or contractors connected to Diadoc or not by sending a list of contractors to diadoc@skbkontur.ru. In return mail you will receive a list of contractors already connected to Diadoc. And if you want to connect to Diadoc and operate real e-invoices and e-documents — just tell us filling in application form.