Grab is the leftmost option on the Intent toolbar. It can be selected by double clicking.
When you grab somebody, you will start by pulling them after you. Click on them again to grab them aggressively, again to put your hands round their neck, and again to start strangling them to death. To stop strangling or grabbing somebody, click Drop or the flashing red button above the inventory. This same button can also be used instead of clicking to progress your hold on a person.