You can use user groups to give the personal discount for user. To include user to one of 5 user groups use IPrice user group manager: Joomla admin panel->Components->IPrice minimart->User groups.
To configure discounts for user groups see the chapter "Discounts"
By default, user groups have names "Group 1", "Group 2" and etc. You can rename user groups by editing language file located on server folder /administrator/languages/xx-YY/xx-YY.com_ipricecalc.ini, where xx-YY - Joomla abbreviation of your language. For example, You need to edit file /administrator/languages/en-GB/en-GB.com_ipricecalc.ini for English language.
Open language file for editing and find variables GROUP 1,GROUP 2, GROUP 3, GROUP 4, GROUP 5. Set the name of the group in the right part of expression GROUP X="Group X".