If memory serves correctly (and that's by no means as sure a thing as it used to be), markers are tied to the timeline, not to events or groups. However, if you need to add more time somewhere in the project you can use Insert / Time to insert an empty space and the markers beyond the insertion point will move as well. If you have extra space left over you can double-click in the space to hightlight it, make sure ripple edits are enabled, and delete the empty space. The markers will move back along with the rest of the events.