When Deadpool is in the hallway talking
he makes a joke about Fox springing for a few more X-Men and the camera pans to show Beast closing the door to get rid of the noise.
Who else was in the room?
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While most sources seem to have caught a couple, DigitalSpy seems to be confident, from their wording, that they saw everyone. Den of Geek's incredibly detailed commentary on Easter eggs gives the longest list of 6 characters:
This seems to be confirmed by the following still:
This lineup seems to be a shout to the X-Men: Dark Phoenix characters and the current X-men team in the X-Men Cinematic Universe.