Question

why is group 17 is called halogen group

Answer 1

this is because halo means salt and gen means former 
as group 17 elements form salt they are called halogen

Answer 2

In Greek 'halogen' mean salt producer. A salt consists of anion and cation. For example common salt (NaCl) consists Na+(cation) and Cl-(anion). The elements of this group form the anion of salt easily, hence they are called Halogens.