Ehm...for some reason, my Like button is missing today, so instead I've been Lurking around this one since you raised it, wondering if anyone would venture an out of the box suggestion (such as an Assignments report)...but no takers, so I'll bite.
Depending on the nature of your Tasks (specifically, if they're typically more than 1 day in duration), I am not aware of a way to accurately report on the rollups you are asking for within Workront reporting: % Allocation, Schedules, PTO, FTE, Lions, Tigers, Bears...oh my! It's Complicated. So complicated, in fact, that we have (over the years) created our Capacity Charting, Heatmap, and (most recently) Resource Contouring solutions to make managing and reporting on such matters easier.
In case one or more of them resonate with you, I invite you to review them at your convenience, and drop me an email at email@example.com if you'd like more info.