Session History Horizon Control

Session history horizon control techniques are described in which participants in a communication session are able to selectively control how much history of the communication session is shown to individuals added into the session. A representation of a session record (e.g., a message thread) may be exposed for rendering by communication modules used by participants for the communication session. Responsive to addition of participants, the communication module(s) may be configured to expose a history control element in conjunction with the session record to enable control over portions of the session record exposed to people added into the session. The history control element for a particular participant is configured to visually identify portions within the session record that are accessible to the particular participant. The history control element may also be positioned in different locations in the session record to modify exposure of the messages to the particular participant.