There are two ways of doing that:
1. Directly from |My moey!|:
- Swipe any expense
- A shortcut will appear, allowing you to send it to the Group of your choice
2. Inside a Group:
- Use the option to add an expense
- Choose if you want to add manually or choose an expense from your timeline
An expense can only be associated to a person, but it can be divided by all or some of the members of a Group.