GroupDetails: change info text (?) in invitation section

Sorry @christian.hansen, I was 1) a bit to late to suggest this before branch was merged 2) I tend to suggest too longs text if I don't see them myself embedded in the interface.

Please change info text to "Copy and send links to people you want to invite to your group. Invitees must be registered users".

Otherwise the makeover of the GroupDetails is great!