Facebook has recently added the ability for pages to join groups. When I enter a group as myself, I'm presented with the choice join the group as one of my pages. However, the choice is just for a few of my pages and not for all of them. Why is that?
Groups may designate whether they allow "Pages" to join, or only "Members". The difference being that if you are applying to join as yourself, or as a fan page/business page/etc.
If the group does not allow Pages to become members then this option will not even show up.
According to Facebook this is due to Pages abusing their group memberships by spamming the groups members.