You can invite people to your group by sending them the website address of your group's profile page or clicking on 'invite people' button on the right-hand side of your Club/Group Profile.
Note: The 'Invite by email' link will open up your mailing app. If you do not have a mailing app installed, you can send the direct hyperlink to your Club/Group profile instead (just copy and paste it from the address bar when you are on your Club/Group Profile Page).