Before now you had to opt in for the Client Beta and get a secret handshake. At the end of March they significantly updated In-Home Streaming, and invited all group members.
Now you don't need to be part of the Steam Group. In-Home Streaming is far from perfect as Valve continues to refine its architecture and add support for more games.
Valve hope this will lead to a large influx of testers so they can keep putting it through its paces. In-Home Streaming is a great complement to the SteamOS machines due this year, as it means PC gamers needn't spend huge sums of money but instead rely on their PC rigs to stream games to the mini-PCs in their living rooms etc.