Hi @RobRoy,
When saying table, do you mean SharePoint list ?
Would it be available for you to share some data referene for those two lists, or data sample?
In addition, which control that you used to display the 'table' data?
For better understanding, would it be available for you to explain a bit for "the intended purpose is to create an additional search gallery that finds event dates by the people who attended them."?
For the Addcolumn function, for example:
SortByColumns(Filter('Testing list', StartsWith(SText, TextSearchBox1.Text)), "SText", If(SortDescending1, Descending, Ascending))
Here the Filter function returns a table, and the addcolumns function should accept the bold part as the table parameter, so if yo uwould like use the addcolumns with the formula above, reference below:
SortByColumns( Addcolumns(Filter('Testing list', StartsWith(SText, TextSearchBox1.Text)), ColumnName1, Formula1), "SText", If(SortDescending1, Descending, Ascending))
Please reply back if you need any further assistance.
Regards