Complex sentence: Until mid- 2009, the blog "Overcoming Bias" was a group blog co-edited by Hanson and Eliezer Yudkowsky, it was his personal blog until 2012, when he announced that Katja Grace and Robert Wiblin would be joining as editors and contributors.
Split into simpler sentences: Until mid- 2009, the blog "Overcoming Bias" was a group blog co-edited by Hanson and Eliezer Yudkowsky. After this, it was Hanson's personal blog until 2012, when he announced that Katja Grace and Robert Wiblin would be joining as editors and contributors.