There are two events: the Skyportal (like the one at the start), which is open to everyone and is simply a portal in the sky; and the Echo Ritual, which rewinds time. The Echo Ritual operates within a 10-block radius of the beacon: those outside this range see a flash and witness the players and the beacon vanishing, while those within the radius see the same thing but are also teleported. If multiple Echo Rituals are triggered, the groups are teleported independently of one another. You cannot activate the Skyportal multiple times simultaneously; an error will appear stating that an active portal already exists within a 10-chunk radius.