This issue may be related to the Steam Client; as a result we expect a fix to take some time to deliver.
We’re currently experiencing an outage in the Steam backend for Online Party after a Steam Client update on March 4th. As a result, Online Party is currently unavailable.
We’ve engaged in discussions with Valve on the issue and hope to have a fix available with their assistance. However, given that this issue may be related to the Steam Client, we expect a fix to take some time to deliver.
We’ll post an update here once the issue is resolved.