There is no guarantee that the administrative law judge will allow your witness to testify or provide input.
Yes. You can bring a witness to your Social Security Disability Insurance (SSDI) or Supplemental Security Income (SSI) hearing. However, there is no guarantee that the administrative law judge will allow your witness to testify or provide input. This is entirely up to the judge.
The judge at your hearing will be making a decision regarding your disability mostly on the basis of your medical records and hearing your testimony. Having your own witness at your hearing is probably not going to help you.
On many occasions, the administrative law judge will decide that a medical expert, vocational expert, or both, are needed at a disability hearing.
These expert witnesses are contracted by the hearing office to attend a hearing and provide testimony. These witnesses are not there to help you or to hurt you.
Vocational experts will provide input as to what jobs might be available based on your education, work experience, and, your disability.
Medical experts provide input mainly regarding your medical records. Judges are not Medical Doctors and will sometimes need input from a medical expert to help him/her understand details in your medical records.