From the main menu > Tap the Contacts tab > In the search bar enter your friend's full name, username or email address > Press Search
Once you have located your friend, select their name and tap on the "+Add" button. This will add them to your contacts. To start a chat with your new contact: Go to Chats > Press blue + button (bottom right) > Select new contact from list and that will start your chat!
From the main menu > Go to your contact list > enter full name, email address or username of your friend in the search field > Press Search > Select their profile > tap "Add to Contacts"
Once you have added your friend to your contact list you can start a chat with them by clicking the Compose button (bottom right) in Chats and selecting the friend you just added from your contacts.
*Note: if you are searching for your friend by their name and cannot find them, please try searching by their username or email as they may have "Privacy Mode" enabled.