You'll need your EIN letter to join Rho.
If you don't have an EIN letter on hand, you can easily get one faxed or mailed by calling the IRS Business & Specialty Tax Line toll free at 1-800-829-4933 between 7:00 AM and 7:00 PM local time. When you reach them, request a 147c letter. The request is free and typically processed the same-day.
If you do not have a fax number, you can request that the IRS fax it directly to Rho at +1 646-455-3240. Please alert your business banker that it's on its way so we can look out for it.