The disclosed system provides an inline messaging capability for multiple participants in a messaging thread. The disclosed system allows a participant in a thread to insert an inline message, which is a message that appears between any two previously received messages selected by the participant (rather than limiting the participant to inserting a new message at the end of the messaging thread). The disclosed system assigns a hierarchical sequence number to each message that enables the system to display messages in a messaging thread in a desired order. The disclosed system also enables the addition of one or more new participants to an existing messaging thread with delivery of previous messages in the thread to the newly added participant or participants.