To add or edit Inventory Groups go to SETUP | PARAMETERS | INVENTORY GROUPS. Inventory Groups allow you to define categories for your inventory items which will appear in your drop-down list in Add Charge and in Reports.
Once you create an Inventory Group, then go to an Individual Inventory Item and assign it to a Group. See Inventory Items.
There are three Inventory Groups that have already been setup for your property and it is important that you do not modify or remove them: RENT, PHONE and MISC. You should add an Inventory Group for every category of item(s)/service(s) that you offer that are not covered by an existing group. Examples include, Restaurant, Spa, Gift Shop, Room Service.