Yes, you can use the map command. Here is the short hand version for what you are trying to do:
roasterroster.map(&:user) # if roasterroster = [r1, r2, r3] then the output will be [r1.user, r2.user, r3.user]
You can read more about the map function in the Ruby Array API: http://ruby-doc.org/core-2.0.0/Array.html#method-i-map