Every Lumi User has a Personal Vault, and you can create a Team Vault to limit the access to specific items to a group within your Lumi Project.
This is your Vault.
Press the plus button in the Vault heading and search for users in Lumi to add.
To remove a user from the Vault, press the bin icon next to their name.
You may want to change the owner of the Vault as they are responsible for adding and removing users. As the owner, you can edit the owner by pressing the pen icon next to your name, then the cross next to your name, search for another user and "Assign Owner".