Does Differential Amplifier Emitter behave as ground in AC-analysis?
Consider a single emitter follower - the output impedance at the emitter is low (usually just a few ohms). This is because the emitter "follows" the base voltage when the BJT is properly biased.

Tying two emitters together means that both emitters present just a few ohms to each other. It follows from this that you can regard the emitter junction as being a fairly hard AC signal point and can, when ignoring DC levels be regarded as ground.