As a clan manager, I may be of some assistance. Here are a few things that you must make sure to do before you can invite someone:
- Add them as a friend
- Make sure they haven't blacklisted you or vice-versa. You can check this by trying to send them mail.
- You haven't already sent them an invite. You can check this by going to 'Manage or Remove Members'
That's all my advice for now. Hope it helps :)