After the first invitees refused to come to the banquet, the king said "Just go ask anybody on the street." They did that, and the wedding hall was filled with people who had had no reason to believe that they would ever have been included in such a celebration.
One guy was so unprepared that he showed up in the wrong outfit. The king was furious, "Throw him out right now."
I am comfortable with the interpretation of the beginning of this parable as describing how some people refused God's invitation and about how God would reach out to people that might not seem to important by worldly standards. I am much less comfortable with the king's reaction to the guest who is dressed inappropriately.
Grace can get us in, but we will still be judged on what we do with that gift. Those on the first list were too smug or too busy to even show up. At least one on the second list couldn't do what the host required.
Some commentators suggest that Matthew is describing the destruction of Jerusalem by the Romans. This could be seen as an echo of Jeremiah's prophecy of the oncoming destruction by the Babylonians (Jeremiah 25:8-14). (thanks as always to the NISB.)
Lectio Divina: Matthew 22:14