I plan to travel to America in the near future. Can I travel alone directly? Is it easy to get a visa if you don't join the group?

This is called free travel or tourist visa. You can apply by yourself, without looking for a travel agency. If you have been to the United States and Europe before and have a visa history on your passport, it is much easier to apply for going to the United States again. If it is the first time to go to the United States for free, it is easier to be rejected. Unless you can prove that you have no tendency to emigrate.