https://ngs.pso2.com/news/events/joinalliance0915 Says you need a full alliance party.
https://pso2.jp/players/event/sonic/team/ Says you need 1 other alliance member in the party.
https://ngs.pso2.com/news/events/cmpgnfriendslist0901 Says you need a full party of friends.
https://pso2.jp/players/event/sonic/friend/ Says you need 1 friend in the party.
I believe these to be errors on the English site because even in game the translation for the Tails one it says you need just one friend and that absolutely works as that is how I got it.