This is pretty cool because if you backup the wallet (which you were already doing) you also backup the messaging keys (which wasn't true before).
In the new Group Wallet, we eliminate the need for Matrix and separate chat keys by repurposing the wallet’s Output Descriptors to derive a single, shared encryption key for the group. These Output Descriptors are contained in the wallet configuration file (also known as the BSMS file). By backing up this file, you can reconstruct the group wallet and its encrypted communication channel whenever you need.
This is powered by a two-phase encryption scheme — asymmetric first, then symmetric — and it’s all handled automatically behind the scenes.