Can a sequence start when someone joins a segment?

Yes. This is built in, and it is the most common thing people build a workaround for before they find it.

Sequences have an Entry Point step with an Entry Triggers section. Two of the triggers there enroll people automatically:

• Segment trigger — a dropdown of your existing segments. Anyone who starts matching the segment gets enrolled.
• Tag trigger — a single tag. Anyone who gets that tag gets enrolled.

The sequences documentation covers both, along with the other two ways in (adding people by hand, and the Sequences API).

You probably do not need a date-based segment

A common workaround is to build a segment like “created in the last 1 day” and point the sequence at that, as a stand-in for “recently added.” You can skip that.

The Segment trigger fires when a subscriber starts matching the segment, not when the segment is evaluated on a schedule you have to approximate. So the segment can express what you actually care about — has a tag, opened something, carries a particular custom data value — and enrollment follows from that directly. Broadcast checks every 15 minutes for subscribers who have newly started matching.

That also removes the failure mode the date proxy has: if the sync and your “last 1 day” window ever drift apart, people fall through the gap or get enrolled twice.

Which trigger to use

Use the Segment trigger when membership is a consequence of data you already hold: engagement, custom fields, subscription date, tags, or any combination. Segments are rule-based and evaluated continuously, so this covers most cases. See segmenting subscribers for what the rules can match on.

Use the Tag trigger when enrollment is an explicit act rather than a consequence — your app tags someone at checkout, or you tag them by hand. Tag the subscriber through the Subscribers API and the sequence picks them up.

Two things to know about the Tag trigger: it takes a single tag, and it is an exact match. There is no multi-tag trigger. If you need “any of these three tags,” apply one shared tag alongside them and trigger on that.

Worth knowing

Enrollment is not instant. The segment sync runs every 15 minutes. If a sequence has to fire the second something happens, use the Sequences API to enroll directly from your own code rather than waiting for the sync.

Sequences can hand off to each other. A sequence step can add or remove tags and move subscribers to another sequence, so one flow can enroll someone into the next without you wiring anything up. See branching a sequence.

Removing people works the same way. Removing a tag can take a subscriber out of a sequence, and unsubscribed subscribers are dropped from every sequence automatically.

Trying to trigger on something that is not a tag or a segment? Tell us what the event is — if it is not covered, it goes on the roadmap.