the joomla user needs to be assigned on the Contact form, not the invoice form.

if you open the contact in the contact manager, you will see the option to "search joomla user".. that's the one you need

the joomla user needs to be assigned on the Contact form, not the invoice form.

Alright, but why is there a user section in the invoice form? It was confusing to me as it looked like I could assign a user directly form the form. What is the purpose of this redundant form?

if you open the contact in the contact manager, you will see the option to "search joomla user".. that's the one you need

Ok, Now that I have made a contact, how do I get that contact into the invoice that I have already created? The section of the invoice manager is not responding to any input that I try to put in the boxes no matter what radio button I have selected.

it's not redundant at all. this fields (the user fields in the invoice form) are for that invoice only. that means that you can import them FROM the contact saved, but will be saved on the invoice separately. this means that you can have different contact details on different invoices, even if they belong to the same contact.. do you know what I mean? (our users find this very helpful!)

for instance, imagine that you have a customer that changes his address. if you changed this on the contact form and this was applied to all the invoice for that contact, old invoice will change too.. you don't want that! with our system, you can change the contact details without necessarily changing the invoice contact details... no redundancy

in any case, it appears that maybe there's some error in your installation that makes the "search" buttons not work (hence the confusion)
when you search for a contact, you should see some options loaded, and you can pick one.

if that isn't happening to you... could I take a look at your site to try to figure out what's going on? could you maybe send me joomla access at support@ joomlathat.com ?

best,
Germi