1. Create a new group (say ‘London’) as shown in the video here

2. Tap on that ‘London’ group from the home screen

3. Tap on the pencil icon on the top right to edit the group

4. Tap the ‘search’ icon on the top right and enter ‘London’ and then tap ‘search’ on the keyboard

5. Tap ‘Select all’

6. Tap ‘Update Group’