You can already hide conditionally in Jet by using IF statements in row 1 or column A, and while this is useful, there are a few times where you might want to see the hidden data.

I have a report that expands out columns for the last two years, and would like to group all that data except the last three months. Currently I have it set up so that it groups the entire set of columns and then just ungroup the last three manually.

The way I see it working is you'd add +Group to A1 and then just use Group to group the rows or columns in the first row/column.