All of the new Verizon plans are, effectively, "family plans". You just add a line (you can have up to 10). Each phone line is $20; and the lines all share the data bucket you purchase (S, M, L, etc)
The line access fee of $20 assumes the phone is yours with no contract involved. What plan are you on now? Does the person you want to add already have a phone? With Verizon? Another carrier? A few more details will help us give a better idea of how to best get you to the plan you need.