Maybe I'm misunderstanding what you're looking for, but you should be able to do this already. For the first one, you can select specific group members (a 'group' expense is just the default) and then do the split option. For the second, just select the user and the payback option!
- Exclude people from cost (only 4 of 5 people do the road trip)
- Fully claim some cost (someone needs the car for a day to move some stuff for himself)
Are there other apps like this with a nice iPhone client?