How can I offset this group object (attached) by 2.5 cm and extrude it like a pipe?
“Group” is the wrong term.
That means something specific in Rhino and would not be appropriate for these curves.
If those curves are Joined together into a single, closed, polycurve, you can use the Offset command to offset a new polycurve inside or outside or the original. Because is has internal and external corners, you’ll have to configure Offset to add round or sharp corners.
Then you can Extrude them to make your tube, or use any number of other surface creations tools.
Probably the best thing is to take the time to go through the Tutorials in the User’s Guide (from the Learn page), and a good, comprehensive set of tutorials so you can learn how to model using Rhino tools.
Good luck