Agree with Irishpilot, when I unfroze my ATPL, no signature required and no letter from the company, some people carry their logbooks and ask for it to be signed at the end of a flight.
I only get it signed after a line check or something like that but stll no requirement for it.
As far as logging the hours go as co-pilot when it is your handling sector it must be logged as P1 U/S.
Hope this helps.