Dependent means spouse or unmarried children under 21.

There are many persons whose immigration petition is pending that are unmarried and would like to get married at some point and would like to get their spouse to US as soon as possible. However, it is very important to plan ahead and take appropriate steps in order to avoid any delays.

Procedure described here is allowed only for immigrant categories that allow derivative status for spouses and children. e.g., employment based immigration. If you are beneficiary of family based immigration such as unmarried children of US etc, you can't get married to bring your spouse as described here.

Marriage/Child birth after becoming permanent resident
If you get married after you become permanent resident (your adjustment of status application approved), you can only bring your spouse based on Family Based Immigration category F2A which may take many years.

Marriage/Child birth before becoming permanent resident
If the spouse or child is in the United States on a valid visa, the individual derivatives may file their Form I-485 adjustment of status applications concurrently with the Form I-485 for the principal applicant or any time before I-485 is approved.

Exception: If the spouse/kids are on a J-1/J-2 visa with a 2 years HRR (Home Residency Requirement), they may not adjust status to permanent residency unless they have either served the HRR or have obtained a waiver of the HRR.

All single H1/L1 applicants who maintain their non-immigrant status by not using EAD or traveling on AP, can get married after filing adjustment of status application and bring his/her spouse on H4/L2 before the adjustment of status application is approved.

Also see EAD/AP vs. H1

Even though Part 7 in form I-140 indicates to include the dependents, it is fine to get I-140 approved and then get married and then apply for adjustment of status.

If the person is already in US on valid nonimmigrant visa, it is also possible to get married in US and file adjustment of status for spouse.

If the spouse or child is residing abroad, the person adjusting status in the United States should file the Form I-824, Application for Action on an Approved Application or Petition, concurrently with the principal's adjustment of status application (or any time before principal applicant's I-485 is approved) to allow the derivatives to immigrate to the United States without delay if the principal's adjustment of status application is approved. The fee submitted with the Form I-824 will not be refunded if the principal's adjustment is not granted. Such dependent would go through Consular Processing.

Procedure for filing I-485 for the dependents is same as the procedure for primary applicant, except G-28.

Additionally, following documents are required:

• Application for dependents can be filed along with the primary applicant. Or with evidence that the application is pending with USCIS or was approved, or with evidence that your spouse or parent was granted permanent residence based on an immigrant visa.
  
  
• If you are the spouse, copy of your marriage certificate for spouse and copies of documents showing the legal termination of all other marriages by you and your spouse
  
  
• If you are the child, copy of your birth certificate for child and if the primary applicant is not your parent, submit copies of evidence (such as a marriage certificate and documents showing the legal termination of all other marriages and an adoption decree) to demonstrate that you qualify as his or her child.
  
  
• I-134, Affidavit of Support
  Whether I-134 is necessary or not varies from the lawyer to lawyer. Some say it is needed while others say that the primary applicant's employment letter is enough and affidavit of support is not necessary.
  
  
• Category specific
  
  • Asylee
    
    • A copy of the letter granting you derivative asylee status either on the basis of having been included on the principal’s original asylum application or on the basis of having been the beneficiary of an I-730 petition filed by the principal.
      
      
    If you were granted derivative asylee status as the child of an asylee and you are now over age 21 and are unmarried, you should contact the nearest asylum office and request information on filing a "nunc pro tunc" asylum application (using Form I-589). You may apply for adjustment of status after you have been physically present in the United States for a period of one year after the date you were granted asylum status.