I believe you can have multiple connections to the same ground. Look at this thread for a little more information. Connect Two Circuit Playground Classic boards to App Lab - #3 by bpenney Also, if you run into other issues on this level, please report back and/or let support@code.org know. I know there have been a few issues with this activity not fully functioning correctly and last I heard the engineers were trying to troubleshoot it.
I wish you the best!
Mike