Errata: I am not allowed to edit the description and I noticed a typo:

In first code example the assert line:

 assert users.every{ it.branch == 'Bob'} 

should be:

 assert users.every{ it.nickName == 'Bob'} 

This line is correct in my second example. Copy-paste error but I hope I have explained it well enough.
(Also please note this is Groovy example, so == is the same as Java .equals())