While he is only a suspect at this point, Ahmad Khan Rahami did not work with anyone when he alleged set the bombs in New York. At this point in the investigation, there is no indication that Rahami was working with other terrorists. There are many radical individuals around the world who are voluntarily siding with militant groups and taking action in the name of justice. Rahami is a suspected radical who acted on his own.
When no group claims responsibility for an attack and there is no evidence that a terrorist worked with others, it is likely that they acted alone. Even so, a thorough investigation should take place to determine if Ahmad Khan Rahimi worked alone. If he was supported or encouraged by others, they too are responsible.
No, to carry out the Manhattan Bombing, Ahmad Khan Rahami must not have acted alone. He was the frontman, who was identified on surveillance video. But during the planning period, there must be others who plant the ideas into his mind, there must be others who help him seek out the materials, and the "foolproof" plan. If he was caught, the people behind the scene may or may not be revealed, but there should be someone.
It is too soon to know that Ahmad Khan Rahami acted alone in the NYC Blast. While the New York Police Department has enough evidence to bring him in for questioning, they do not at this time have all the answers. It is possible that he worked within a group.