First be sure to follow our guide to connecting PCO to your website. Once that is done, follow the outline below.
Go to the page that you'd like to display your groups one.
Add a new Groups Engine Block to the page.
Set the type of block to "Sync From Planning Center" and setting the Planning Center ID for the group. You can find the PCO ID by going to your Church Center Page and selecting a group type. The ID is the last part of the url.
Click save to save the page.