When did Plymouth colony join Massacusetts colony?

If you are asking when Mass became a royal colony it was in 1691 and Plymouth was established in 1620. In 1630 the Mass Bay Co got a royal charter for most of present day Mass and New Hampshire. The stockholders made John Winthrop the first governor. So, there is a couple of dates here that you could use and still be correct. I think that since Plymouth was established first that they didn't "join' Massachusetts, but it developed around them.