The scoreboard teams join command used to allow a list of player names. e.g.

scoreboard teams join red Aaaaa Bbbbb Ccccc

The new team join command throws an error if a space separated list of player names is supplied. Specifically,

team join red Aaaaa Bbbbb Ccccc

gives "Incorrect argument for command at position 20: ...ods Aaaaa <--[HERE]".

Trying to supply a comma separated list just concatenates the names.