抓阄 is a separable verb (离合词) meaning 'to draw lots' — you pull a slip of paper from a group to make a random decision. The verb 抓 and the object 阄 can split when you add a measure word, a modifier, or a result complement: 抓一个阄 ('draw one lot'), 抓了阄 ('drew lots'), 抓完阄 ('finished drawing lots'). You can also put the number of participants or the purpose before 阄: 抓个阄决定谁去 ('draw a lot to decide who goes').