Can Sybil join Sam’s party?
No. Sybil’s role is in the telescope story and ending choices rather than the companion party.
Follow Sybil’s telescope story before the rooftop ritual if you want to keep the Promise ending available.

Ask the astronomers about Sybil, then investigate Apartment 12 for the telescope route.
The Promise route depends on Sybil surviving. Attacking her after restoring her memories closes that ending route. Save before choosing how to answer her request.
Sybil is a story character, not a recruitable party member. Her apartment, old telescope and connection to the astronomers change what Sam can say on the roof. The regular Perfect Ritual checklist alone does not prove the Promise route is ready.
No. Sybil’s role is in the telescope story and ending choices rather than the companion party.
Apartment 35 is on Floor 3. The telescope route also takes you through her earlier Apartment 12 story.
Restore the telescope, give it to Sybil, make the promise, complete the four true offerings and raise Sybil during the rooftop encounter. Do not attack her.