To join your friend's guild, you need to press the Guilds icon at the bottom of the main screen. Press the Search button in the bottom left-hand corner of the window that opens.

After you enter the name of your friend's guild, a list of guilds matching the search criteria will appear.

If the list hasn't appeared, perhaps you should check if the name was spelled correctly, or if the guild you're looking for is already full and can't be displayed.