There is currently a limitation in our system where users who haven't visited the community pages are not appearing for group owners to invite. To visit the community, please ask your friend to click on the community tab on the website or in the iOS or Android apps. You can send them this information via private message or write on their wall with a link to the group. (If it's a private group, we suggest a private message.)
Please note that invitations to groups don't currently show up in your main MyFitnessPal notifications but in the community-specific notifications located under the bell icon on the community pages.