At the end of the Alabastia arc the straw hats said goodbye by showing the "x" on their wrist. The "x" on their wrist represents their friendship. Do Robin, Franky, and Brooke receive the same mark on their wrist?
The mark that the Straw Hats made at the end of Alabasta arc has probably washed off by now as we haven't seen it in the following arcs. There is no subsequent reason for the new members to make them as the meaning was related with the events in that arc (at least that is how I personally saw it).
This does not mean that at any time, Oda could show that they still have the mark barely visible and the new crew members (after Alabasta) will have their own too.