You can do this via different routes: 


  1. Within the ‘mailing lists’ page, click ‘+’ to add a new contact.
    Fill out the form and assign them to relevant groups. You can add them to multiple groups if necessary.
  2. For single contacts: Within the ‘mailing lists’ page select the contact, click 'add to group' and then select the appropriate group
  3. For multiple contacts: Within the ‘mailing lists’ page shift and click multiple contacts, click 'add to group' and then select the appropriate group 
  4. For multiple contacts: Within the ‘mailing lists’ page cmd/ctrl and click multiple contacts, click 'add to group' and then select the appropriate group