As far as I know, Tobi didn't come to limelight until Sasori's death (I am talking as per the chronological order of the telly series). He teamed up with Deidara after Sasori's death. He knew Nagato and his team long back but he was not proactive with the organization at the beginning of series.
Why didn't he volunteer himself? Is it because Akatsuki work in pairs and he didn't have anyone? Or was he waiting for the right moment to join with the organization?