If you are a U.S. citizen and you want to help your family members become legal residents in the country, you have the ability to do that. As with anything immigration-related, there are rules and requirements you and they must meet.
The U.S. Citizenship and Immigration Services explains that it always sets aside visas for relatives of citizens, but immediate possession is not always available. It depends on the relationship.
The U.S. government defines immediate relatives as a parent, unmarried child under the age of 21 or a spouse. If the relative you wish to seek a visa for falls into one of those categories, then he or she will have no wait to secure one. You will need to file the proper forms, Form I-485 and Form I-130, but the government has these visas ready to go.
You may still bring other relatives to the country, but they may face a waiting period. The government will assign preference based on the relationship. These categories do have limits on the number of available visas, and your family member will have to wait until a visa becomes available.
Behind immediate relatives, are your children that are over the age of 21. However, they must still not have a spouse. Married children are at the third tier of consideration. The second tier is for the family of green card holders and permanent residents. The lowest level of consideration is for your siblings.
When planning to help a family member get a visa to become a permanent resident in the U.S., you have a lot more ability to bring family here than someone who is not a citizen, but that does not mean your relatives will immediately secure a visa. That depends on your relationship and the current state of the system.