Hike intelligently synchronize with your Phone Book and show all your contacts on hike. So if you want to add a contact on hike, just add them on your phone-book and it will show up on hike. It's as simple as that :)
Contacts who are on hike:
Contacts who are on hike will show up under 'People on hike' or if you have added them as a favorite, then under 'Favorites' section and contacts who are not on hike will show up under 'SMS contacts' section.
Receiving message from a unknown number:
If you receive a message from a unknown number, you will see an option to add that number in your contact, or block the number, if you intent not to receive any more messages from that number.