In the Friends section, tap Manage Friend List → Add Friefirnedsnd → add your friend's Forest account (the email address they registered with)
Connect your Facebook or Weibo account to find friends who are also using Forest.
In Forest's Settings, go to your account and turn on the option "Allow people to send request from your profile". After you turn this on, people can add you when you're planting trees together! (You can only add other users if they have also turned on this option)