Blizzard made a change after the WoW: Forever Beta and announced it on their podcast. When you join a group in WoW: Forever, you don't automatically phase to the same layer as them. Instead, players will have to click an arrow on the top left of their chat window to manually trigger the phase.
This can be useful when you're out in the world and don't want to trigger a phase change immediately! This popup will occur even if you're in Town doing nothing, so if you don't see your group, make sure to check for this arrow!
