When Argos is installed, all users in the Default group have full access to the Argos Manager. It is common to remove access rights from the default group and instead create a specific group that will be able to access the Manager.
Since Argos permissions are group-based, to allow Manager access to a specific user, just move them into a group that already has the desired permissions level, or create a new group and grant it specific access.
Open the Argos Manager and click on the Users tab on the right hand side.
Double-click the user group you'd like to edit
Choose Can Administer Argos Manager (full permissions) or Access specific parts of Argos Manager