It is assumed that since User 8397/37 has one of the pendants in their possession, they know about The Device and possibly helped created it along with Ben Matheson, Rachel Matheson, and Grace Beaumont.
User 8397/37 sets up an online chat with Grace, so it is assumed that he/she is in possession of another pendant. Grace explains to this person that a group of militia were at her house. The user quickly asks her if they found it. She said they did not, the person asks what they should do next.
Later, when Randall breaks into Grace's house, she has just enough time to warn User 8397/37 that he has found her.
User 8397/37 could be John Sanborn, as John was aware that Randall had captured Grace and he had a pendant.