First, from your dashboard, deploy the instance you want to add another user to. Permissions for instances can currently only be edited once the instance has been deployed. Once it's deployed, go to your control panel, and select Manage Instance, on the left side menu.
From there, click on User Permissions.
Then, type the username of the person you're wanting to add to your instance, edit the permissions to your liking, and click save.
Users will need to have a nodecraft.com account before you're able to assign them access. Users can register for free at https://nodecraft.com/register
To remove a user, Just click the remove button underneath action and re-save the permission changes.