登门 literally means 'to step onto the doorstep' and is used to mean 'to visit someone's home'. It is a verb-object compound (离合词), so you can split it by inserting aspect particles (了, 过) and frequency complements (一次, 两次) between 登 and 门: 登过门, 登了两次门. The person being visited is usually indicated by context or with set phrases like 登门拜访.