@ZowieS
Please try the below to genarate your collection that displays user , sum of days:
You add this formula on the visible of your screen or on the start property of your application
ClearCollect(list1,AddColumns('Sharepoint List',"User",CC.DisplayName));
// Where CC the your User Column name
ClearCollect(list2,GroupBy(list1,"User","UserGroup"));
User Group is your grouping name you can use as it is
ClearCollect(list3,AddColumns(list2,"SUM",Sum(UserGroup,DAYS)));
//SUM is the new sum column that will be displayed on list3
//DAYS is the Days Column in your list
list3 can be used now as your table datasource.
If this post helps you with your problem, please mark your as Accepted solution.
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MG (Naturally Curious)