How to make a contact group in itunes?

Basically me and my dad share the same computer to sync our iphones to itunes and we both sync contacts, however everytime we sync our phones now it adds his contacts on top of the contacts on my phone and my contacts go on his as well, is there any way you can create a group or something because on my itunes under the info column (where the contacts part is located) there's a "selected groups" option however its blanked out (i can't click on it) so how do i make 2 contact groups to seperatly put his contacts in one and my contacts in another so they don't clash and all go on one phone at once?

No idea