In the latest Steam beta client you can Remote Play Together with anyone, no Steam account needed, just by sending them a link. The blog post outlining the latest beta client update shows you know to opt-in to the new program. For those with it installed, while playing a Remote Play Together game you can simply bring up the Steam overlay and grab a link from there.
Others can then use that link to join your game. If they don't have Steam on their computer, they'll be given a download for the Steam Link app. You can invite just one person this way, though additional Steam Friends can be invited using the same link or through the Steam friends list as normal.
If you're not familiar, Remote Play Together is a feature on Steam that lets you bring others into local multiplayer and hotseat games even if they're not physically present. It's a godsend for lots of games, especially those hardcore local multiplayer games that are hard to get four people in once place for, or for hotseat on wargames, where multiplayer service is rudimentary or limited.
We're of the opinion that Remote Play Together is one of Steam's best features. The ability to turn a local multiplayer game into an online multiplayer game is powerful—and now you can do it with friends and/or grandparents whose 15 year old potato PC can barely run Steam.