If an angel wants to exit, do they have to get permission from the rest of the pool?

Angels have the option to unilaterally terminate the trust for their own shares, but this is subject to a notice period as stated in the Trust Agreement. However, for the termination to take effect, the angel must then agree to the underlying shareholders' agreement as a direct shareholder. This usually involves the target company and its shareholders (including a group of angels) agreeing to the accession of a new shareholder and any necessary amendments to the SHA.