If you sent your vendor an invitation to receive ePayments from your company, and then learned that they already use Bill.com, don't worry! You can connect with their existing Bill.com account - they don't have to create a new one in order to connect with you. All you need is their Payment Network ID, which they provide to you; they'll find it on their Home Page.
Once you have their Payment Network ID, overriding your original emailed invitation is easy:

Hover your cursor over the Network tab, and click Connections.
Click "Add New Connection"
Enter your vendor's Payment Network ID and click Find.
Click Connect
Select the vendor from the drop-down menu, and click Send.

Your new invitation to connect will be sent to your vendor. Once they connect with you through the Network, you'll be able to pay them via ePayment.