How to select, programmatically, only rows from a grid that are currently displaying on the page

Hi,
I have created a page with a grid at level 1 with checkbox. I add a push button to select rows from the grid. however, the process selects all rows which could be in thousands. What I want to do is to select only the rows that are currently displaying on the page (which are usually 50 or 100 if I click 'view 100').

Let me know if there is any way or build in function available to do this.

Thanks in advance

Rehan