How do I share items I post on Facebook only with members of a custom group I have created in Facebook?
make a 'list' of the friends you want in that group, if you haven't already done so.
When you post the item click the padlock icon and choose 'customize'.
in 'show this to' choose 'specific people...' from the dropdown menu.
This will make a text input field appear, in it start typing the name you gave to the friend list, it should come up as a prediction/suggestion, click on it and then choose 'save settings' - you may want to set this as the default using the 'Make this my default setting' checkbox.
Then all you have to do is post the item(s).