Heck, they don't even need to have a Steam account.
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Valve just rolled out a big update for Steam today. This update, among other changes, introduces the ability for players to invite literally anybody to Remote Play Together. The invited person does not even need to have a Steam account!

This new update will allow those with a Steam account to grab an invite link and send it to anybody that they want. The person that accepts the invite will then be able to play with you as if they had a Steam account of their own. It should be seamless.

To celebrate the full launch of this functionality (it's been in beta for a while), there is a Remote Play Together Sale & Streaming Event beginning March 25 at 10AM (PT). You will be able to watch a livestreaming marathon where you can see Remote Play Together in action and even join some games in progress. The streamers associated with the event can be found in the above image.

The full list of changes in today's Steam update can be found below.

General
  • Added spelling suggestions to Library and Steam Chat context menus
  • Showing size on disk for DLCs under game properties
  • Fixed showing localized game names in pop-up notifications
  • The Steamworks SDK redistributable used by dedicated servers has been brought up-to-date with this release

In-Game Overlay
  • Improved responsiveness of web based UI and of the friends and chat UI inside the in-game overlay
  • Fixed overlay support for 32-bit Vulkan applications on Windows

Steam Input
  • When using Steam Input, active controllers fill in XInput slots in the order they generate input (e.g. the first person to click A will be player 1 in many games
  • Fixed issues with game detecting controllers while the overlay is up on Windows
  • Fix issue with analog axes in the Windows version of Slay the Spire

Remote Play
  • By default stream at 1080p to prevent accidental 4K streams
  • Added an option to stream up to 8K resolutions (results may vary depending on hardware)
  • Added a menubar at the top that gives you an easy way to stop streaming
  • Added a toast when you connect a controller that shows how to bring up additional controller features
  • Improved audio range when using volume controls

Remote Play Together
  • Added support to invite anyone to Remote Play Together

    Now you can Remote Play Together with anyone – no Steam account needed to join your game with the click of a link. Simply launch any of the thousands of games with support for Remote Play Together, then grab an invite link from your Friends list in the overlay. Send the link to your friend on Windows, macOS, Linux, iOS or Android, and they can click your invite to join in the fun.

    For more information, and to share your feedback regarding the new Invite Anyone functionality, join us in the Steam Remote Play Group.
  • Inviting a friend will not auto-start a voice chat session
  • Remote keyboard and mouse are disabled by default in Remote Play Together
  • Added a connect dialog that gives you information about who is playing and what input they're using
  • Local controllers fill XInput slots first and guest controllers fill slots in order of arrival
  • Fixed issue with game controllers not being detected properly when the game doesn't have focus
  • Fixed bug with controller not being recognized if Xbox Configuration Support was enabled on the client PC
  • Enabled direct peer to peer connection by default (this can be changed in the Steam Remote Play settings)
  • Fixed local guide chord input leaking through to the host computer

SteamNetworkingSockets
  • Fixed bug causing 64-bit Windows games to not attempt a direct connection and always relay using Steam Datagram Relay.
  • Fixed bug causing GetQuickConnectionStatus to not populate m_cbSentUnackedReliable properly

macOS
  • Fixed some popup web dialogs that did not close when triggered by the web page to close
  • Fixed mouse interaction not working with embedded YouTube videos
  • Fixed overlay not responding to mouse input in certain games on macOS 10.15+
  • Re-enabled HW acceleration settings on macOS 11.1+

Linux
  • Added support for the new Mesa single-file shader cache format, reducing disk usage and improving performance when parsing and modifying the cache. (requires Mesa master as of 08101aaaace22)
  • Fixed a regression where the client would recursively follow symlinks in Proton install folders while discovering music files