For now, Xbox Insiders can sign up to this tier for just $1 per-month, but it'll transition to a $14.99 price point when it becomes available for everyone. The idea behind the new tier is to provide access to online multiplayer along with hundreds of Xbox Game Pass titles, but certain games will be held back until a later date.
"Some games coming to Game Pass Ultimate (day one games or other game entries) will not be immediately available with Game Pass Standard and may be added to the library at a future date (can be up to 12 months or more and will vary by title). We'll continue to share with all Game Pass members when games are being added and available to play for each plan."
Those who are interested in joining can head to the Xbox Insider app on console and look for the Xbox Game Pass Standard option. A select number of titles will be available, with more to be added throughout the preview period.
Obviously if you're already an Xbox Game Pass member you won't be able to join, aside from if you're subscribed to PC Game Pass or Game Pass for Console - or Game Pass Core with less than two months stacked onto your account.