As many have noted, you DO NOT need an invite code to sign up Venmo.

Just navigate to a current member's Venmo page and sign up from there.

Here are some links collected from the posts above:

https://venmo.com/DanielKahn
https://venmo.com/iqrammusic

Or go to https://venmo.com/ and on lower left hand side under "People using Venmo". Just use any one of their page and click on "Signup". Once you sign up, you can "unfriend" them. :-)