Hey @Tadidas!
Responding here in addition to your DM so more people can access the answer in the future. 
For a Closed Beta, leveraging a Player Group would be best. If your product uses EOS, you must have an EAS application configured, approved, and added to your artifact.
Configure your Player Group to grant access to the Dev and/or Stage sandbox and include access to any relevant deployments for this Player Group. If your game uses EOS and a deployment is not included, players may encounter error when trying to play your game.
Next, you’ll need the Epic Account IDs of the players you want to add for your Closed Beta Player Group. Once they’re added, send them a key granting access to the Dev/Stage sandbox build. They’ll need to redeem that key through the EGS launcher. If a player redeems a key before being added to the Player Group, they’ll likely not see the game in their library. To fix this, they will need to redeem a new key.
Another way a Closed Beta could be accomplished would be to have the game in the Live sandbox and distribute keys for that build. So long as the purchasable date is future-dated, no one would get access to the game that doesn’t have a key. This method gives you less access control than a Player Group and may not be ideal for everyone.