Process Group List contains the list of processes that are part of the group sorted in a certain order.
A process group will show an Error result if has one or more Error results in the processes that conform the group.
Process Group is a window, so you can manage permissions for creation of Process Group as you wish: Only System, some clients, some orgs, some roles, etc...
You can not launch executions of empty groups. If you try you will receive this error: No processes on the group: nameOfTheGroup
Prevent Concurrent Execution Co-Exists
Prevent executions of single process and group process will co-exists. Which means that none of them overrides the other one and both can be set at the same time.
Process Group as Source Data
Process Group can be considered as Source Data which means that, working as System Administrator, you can assign a "Group Process" and/or a "Group Process List" to your module in order to distribute them as part of your module.