Yes they'll be limited to the chrome version of stadia. Which supports xinput controllers(basically everything these days) as well as KB+M.
@Lurconis Here's a list of compatible controllers that can be used with Stadia
Stadia-compatible controllers and screens
They'll basically just need access to any kind of PC that can run chrome.
Just make sure to tell them that the experience without chromecast currently isn't the best one (basically none of the Stadia Pro quality features apply... Yet).
I think that's an important message to get across to set expectations for buddies entering the community.