If you and another Pinner are following each other, you can invite them to a group board. To invite someone:
Go to your secret board and click Edit
Enter the first and last name (or email address) of the invitee under Who can Pin?
Click his/her name once it loads
Repeat for any other people you'd like to add
Click Save Changes
What group Pinners can and can't do on a secret board
Only the board creator can make the secret board public. No other person can change this setting.
Note: The board creator can turn off the secret setting at any time without anyone's permission. If the board creator changes the secret setting of the board to visible, all the group Pinners will receive an email notification.
Invited Pinners can view the secret board.
Invited Pinners can request approval for people they'd like to invite to the board. Only the board creator can approve and remove these requested invites.
All invited Pinners can Pin and like Pins on this board. They can also Pin contents of the secret board to other boards. These Pins and likes will not display information about the secret board or link back to the secret board.
There's no "read only" mode for a secret group board. People who accept your invitation to join your secret board will automatically be able to add Pins to your board.