The Invitation is a story mission in South Park: The Fractured But Whole.
Plot details follow.
To start this mission, go to SoDoSoPa. Go upstairs to learn you've been set up by Mitch Conner. Now, you will need to fight an army of ninjas who have been hired to kill you.
Watch out when moving your characters, these ninjas deal massive amounts of damage with bleeding effects. They can also spin and hit several of your allies at once.
After a couple turns, two more ninjas will spawn on the right side. Keep your distance as you pick them off one by one. Soon, two more ninjas will spawn on the left. The white ninjas have the ability to spawn full-health ninjas of their own. You won't be able to take them out without them spawning a couple extras. But, you need to focus your attacks on them. Take them out one at a time while ignoring their spawns for now. With them defeated, take out the rest of the regular ninjas to ensure victory.
Now head to your next mission, The Samaritan Agenda.