Hans
Alright, I got some more information for you:
She must first create a personal bunq account. It's best to choose a Premium account as it will first be in the free trial month meaning you don't get charged for that membership in advance. It is very important she signs up using the same number or email address that you used for the invite. After she has opened her account there is an event in the events list which is your invite for the Joint membership, tap this and follow the steps on the screen.
Hopefully you're up and running in a couple of minutes and you can both enjoy all the benefits bunq has to offer.
Thanks for you feedback and the bunq team will look at how to improve the flow and information provided.
Have a great evening!