• Totally hidden as a Group Item means that, as far as your group members are concerned, the item isn't assigned to the Group at all.  The item will appear as just an individual assignment with no ties to the group. This forgoes any group comment thread about it.
• Visible as a Group Item but hide group performance means they’ll see it as a Group Item, can have a group conversation about it, but not see the overall result: for Metrics that means no aggregate graph will be visible, for Actions and Worksheets that means they’ll have no idea of the overall level of completion of the assignment among their fellow group members, and so on.  You as coach always get to see aggregate performance, of course, but they won’t.
• Visible including aggregate performance means group members will see, for example, how many other group members have completed the assignment and how many have not, and among those completions how many were on time and how many were late, but not which members specifically. The specifics (for example Worksheet answers or individual Metric data) will not be visible to other group members.
• Visible including aggregate and individual performance means complete transparency.  With Actions, everyone knows who’s done it and when.  With Worksheets, all members can read the completed worksheets as submitted by all other group members.  And again, with Metrics, everyone can see the exact reported numbers of everyone else.

Note that you can change these settings for each item later if desired, and the visibility is set for each item individually. Thus, you can adjust if one Metric might be better kept private but another shared with full transparency.