A character can be an in-game guild member without being a member of a Guild Launch guild site (and vice versa). The most likely answer is that while the person is a member of your guild, alliance, corp or other group in-game, he or she is not a member of your Guild Launch site.

After they create a Guild Launch account, they will need to either apply to your site and be accepted or use a Launch Code to bypass the application. Then they’ll be able to see everything properly.