How to add desktop phone to Shared Line Group?

Hi everyone, I'd appreciate some help. I have 3 shared lines in 1 group, with 3 desktop computers. When I try to add another desktop phone to access that shared line group with 3 lines, it won't allow me to add the phone because I don't have an available license. I don't want another new number, I just want to access the 3 shared lines in the shared group. Thank you