Firstly, you must be authorized to make these changes in OnGrid Sky either as a manager-level user, a supporting user, or as a member of a data-sharing group.
If you're authorized, go to the OnGrid Tool's Inputs sheet, click on the "Transfer Customers Between Salespeople" link, a few rows below the "Save Customer Data" link. Press "Yes" for the following message. Select the salesperson you want a scenario or group of scenarios to be assigned to. Then select the scenario you want that salesperson to be assigned to. On Windows, you can select multiple scenarios by pressing and holding the Ctrl key and clicking on the desired scenarios, then releasing Ctrl. Press "Click to Apply Selected Salesperson to Selected Customer(s)" in the upper-right corner then "-- Close -- Apply New Salespeople" in the bottom-right corner.
If you want every scenario on the list to be assigned to your selected salesperson, press "Click to Apply Selected Salesperson to All Customer(s)" instead. Then press "-- Close -- Apply New Salespeople."