This is possible IF you have the latest version of DT Register as it's a new feature. Dynamic / Conditional fields have been available for a long time, but not based on group size until recently.
Each field has the conditional area of options. There is a parameter for minimum and maximum group size. You'll need to create a name field for each group member. Assign that field to "Group Billing" as you'll be using the simple option and there are no Member pages. Then set the conditions like this...
Field: MemberName1
Minimum Group Size: 2
Maximum Group Size: leave blank or 0
Field: MemberName2
Minimum Group Size: 2
Maximum Group Size: leave blank or 0
Field: MemberName3
Minimum Group Size: 3
Maximum Group Size: leave blank or 0
Field: MemberName4
Minimum Group Size: 4
Maximum Group Size: leave blank or 0
etc...
So as the group size increases, the number of name fields that qualify to be displayed also increases. This should do the trick