Jerevoril Reyjor hails from a noble family.
His family was cast out from high society as a side effect of some political squabbling, and he is the last remaining man standing up for his lineage.

As for the invite from Mathis, the motivation ranked first in his mind is that he senses great things to follow this meeting, like the expected upheaval. So he decides to join up with the heroes both to support them and to record their trials. A Bard is a teller of tales, after all.

Of course, being from a displaced noble family gives extra reason for him to become part of the conspiracy. Perhaps it is a wish for revenge, or maybe a fight against the corruption he has seen first hand.