If a group is already been created, we can add or remove parts from the group by using following tools as shown in below image. This wiki explains how to remove parts from group by using the Remove Parts from Group tool.
1. Under Steel workflow, navigate and left click on Groups located at Home > Classification > Groups.
2. Administrate Groups dialog box will open, click on “Remove parts from group” tool.
3. First you need to select the group from which you want to remove a part, then select the part.
4. Now, go to PS Properties to check whether the parts is removed from the group or not.
In this way, we can remove the part(s) from group using Remove Parts from Group tools.