How do you create an approver group with customers in it?

[Ben]

My requirements are pretty simple - I am on the free plan, with my team using all 3 licences.

We need to have a 2 step approval process:

1. Peer Review (1 person)

2. Business approval (1 person out of 3-5)

I've been told here that they just need to be a customer in the project (which they all are/will be) but I have no idea how to create the group with a combination of licensed users and customers!