Find Concurrent Usage

You could create a list of all the hours with an active Usage record.

You can either have a table of hours or generate them in M

let
    Source = List.Generate(()=>0, each _ < 24, each _ + 1),
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Changed Type" = Table.TransformColumnTypes(#"Converted to Table",{{"Column1", Int64.Type}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type",{{"Column1", "Hour"}})    
in
    #"Renamed Columns"

Now we can cross join the Usage data and filter just the hours that are between the start and end.

Create a New Dax Table (under Modeling)

HourUsage = FILTER(CROSSJOIN(Hours,'Usage'), AND(Hours[Hour]>=Hour(Usage[begin_time]), Hours[Hour]<=HOUR(Usage[end_time]) ))

You can then chart the hours and count distinct uids.