I got a response from Ubisoft support, this is what they had to say... This feature is working as intended and cross-invites, Stadia to PC and vice versa, are not yet available in the game.
I am currently unable to add any friends that play on pc or Luna in Rainbow Six Siege. This happens Everytime. See attached pictures. I am however able to add stadia friends with no problem. ...
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I am currently unable to add any friends that play on pc or Luna in Rainbow Six Siege. This happens Everytime. See attached pictures. I am however able to add stadia friends with no problem.