When you click on the " Share" command in the blue command bar you can then choose this command:

You will then see a list of all your share groups - click on one and then you can control the members for that particular share group.

Once you are viewing your share group, if you don't see a particular friends' name that you want to add, just add them to your Friends List, and they will then be available in the Share Group editing screen.

Note - you can also remove a person from a share group by removing them from your Friends list. Click on the gray Visit tab, and then click on "Remove" next to the user's name. They will be removed from both your Friends list and all share groups.

(add name to group; create group; edit group)