You might also find your answer in the LastPass Enterprise Manual.
Why can't I individually share a site in a Shared Folder?
Sites that are in shared folders are cannot be shared on an invidiual basis. You will have to either remove the item from the shared folder to individually share it, create a copy and do the same or restrict the site (see below). This is also true in reverse; a site that is already shared to an invidual or is a part of role must be removed from the share or role, and then added to a Shared Folder OR a copy must be made and then put in the Shared Folder. LastPass will ask you if you'd like to make a copy of the site when you try to move it from an invidiual share to a shared folder.
By using the Restrict feature, you can specify restrict a user to only have access to only some sites in the Shared Folder by doing the following: