You did not specify a particular version of GW you were interested in, so I will tell you where to find a users signature in GW5.5 without any add-ons, like the Enhancement Pack.
The signature does not reside in a local (client side) file, but is resident within the GroupWise system's database for each user. That way, if you login to GroupWise from another PC (using the "GrpWise.exe /@u-?" command), you will be able to still find, use or edit your signature.
The /@u-? is a command line switch that allows you to open a GW account regardless of who you are logged in as, provided that you know that users password (if one is applied to that account).
Log in to GW as the user who's signature you wish to edit.
In GroupWise, select the "Tools" menu item, then select the "Options..." menu item, then select "Environment" from the "Options" window. Select the "Signature" tab. Edit the signature and apply settings as required.
There may be other ways to access this information for each user, but I would recommend that you avoid that if at all possible. Let each user maintain their own signature as they see fit.