On the top right corner, tap the down arrow to open the app’s menu. Tap on “Communities” and select any community you’ve already joined. Near the top right corner, tap the person icon button on the Community’s home page then touch Choose people to share with and type the name of the contact you want to invite.  You can touch the circles or people you want to invite, or in the top left corner, you can touch the magnifying glass to search for a person or circle. You can also add an email address from your contacts. Type in your message on the text box. Once you’re through, In the bottom right corner of the post, touch the send icon.

Summary:
Open the app’s menu. Select a community you want to invite your friend to. Invite your friends. Send your invitation.