Lots of times you aren't the only one who is performing the song that you record. Oftentimes other band members, or paid performers are also performing. When you submit a song to the ReverbNation Premium Song Licensing Program, you need permission from these people to use the song. Having these people sign a Talent Release form is the best way to make sure you have their permission in a written form.
It is not required to get these forms signed, but it is highly recommended. Here is a link to a SAMPLE TALENT RELEASE DOCUMENT you may use at your convenience.