You issued the group add command. This group had some group events (breakpoints or tracepoints) set. When adding a task to this group, the group events will be added for the new task. To add an event, the task must be able to send debugger commands to the remote debugger. In this case, the tasks listed were not in the DEBUG READY state so that remote debugger commands could be issued.
Use the tasks and group list commands to view the state of the tasks. To change the state of tasks, use the halt command and <Ctrl-c> to bring up a pdbx-subset prompt.