Currently (Version 5 beta), in the entry editor, you can add an entry to a group by typing in the group name in the field General: Groups. This is cumbersome and prone to typing error.
A drop-down menu could be added to the field Groups in order to select the group to be added/removed.
Note, that if such a feature is implemented, it should never replace the existing functionality. I can already see this feature becoming a major performance problem for databases with thousands of groups, if it replaces the original functionality.