In the last despair event 3v1's where allowed, That is not the Case this time.
Guardians may drop a white to help a random Host but must not team up together
U can have a random summon with u or be in a random persons world but u can not team up with the random and help kill the invader, if the invader is a dark stalker but it's a random invader im pretty sure u can just go at him