To add someone new to an existing shared objective follow these steps:
1) Go to Objectives Manage View (you will need manage permissions for this)
2) Add yourself to the "Assigned to" in the filter to show your objectives
3) Find the objective which is shared (look for the shared symbol)
4) Edit the objective by clicking on the name of the objective. Click on the share button on the right hand side
5) Add a new user to share the objective with them
6) Notice a comment has been added to the objective conversation to show the objective has been shared with the new user