Error: A server-side database availability group administrative operation failed. Error Failed to add or remove the Failover-Clustering feature. Error: The request to add or remove features on the specified server failed. A DISM session could not be opened. An error occurred accessing the temporary folder C:WindowsTEMP57ACE2DE-4CD2-4F5F-B7A0-93D867A89A12. Ensure that the path to the temporary folder exists and that you have Read/Write permissions on the folder. Error: 0x80070070

Solution: I was attempting to remotely add the DAG member from the ECP on my workstation. I logged into the server and found that the system drive was nearly full (~100MB free). Luckily this mailbox server was a virtual machine, and I was able to quickly expand the drive using VMM. After doing this I was able to successfully add the mailbox server to the DAG.