From version 1.6.0, group membership can be changed in multiple places in the zencontrol commissioning application.
The overview page for a group (navigate to this page by selecting the appropriate row from the group list) lists any ECGs in the group, as well as any ECD that is targeting the group. To remove an ECG from the group, expand the 'Lights' section and swipe the appropriate row left. A dialog will appear, confirming that this is the desired action. To add an ECG, select the 'Click to add' row at the bottom of the 'Lights' list. The next page will allow you to select one or more ECGs to add before saving.
The overview page for an ECG (navigate to this page by selecting the appropriate row from the device list) lists any groups the device is in, as well as any ECD that is directly targeting the ECG. Like the group summary page above, the 'Groups' list rows can be swiped left to delete. Selecting the 'Click to add' row at the bottom of the list will allow you to select one or more groups to be added to.