From VGC: “One of the leading misconceptions about Stadia is that you need to pay to play multiplayer games online.
Thats according to John Justice, Google VP and Stadia head of product, who told StadiaCast in a video interview that lots of consumers think Stadia multiplayer is locked behind a paywall similar to PlayStation Plus or Xbox Live Gold.”
Source: N4G PC Google exec: lots of people dont realise Stadia online multiplayer is free