To assign existing contacts to multiple groups, follow these steps.
1. Go to Contacts and create the groups you wish to use.
2. Search for the contact you wish to add to multiple groups
3. Click on the contact's mobile number
4. This will take you to "Edit Contact'
5. At the bottom of this page under the heading 'Add to Groups', type in the name of the groups you wish to add this particular contact to
6. Alternatively click on "View all groups" and tick the box's of the groups you wish to add this contact to
7. Click Save and your contact is now saved to multiple groups!