Unless he was already running in Weather Underground circles at Occidental he wouldn’t have had time to become a trusted member of their robbery crew. I don’t think he even met Bill Ayres until Chicago, and Ayres wasn’t involved in the Brinks robbery despite being a close friend of those who were.
This article gives the best summation:
They were in the same city as each other, at the same time.
They lived near each other.
The went to school near each other.
They had the same political interests.
Their circles of friends and associates intersected.
By sheer coincidence, a decade later in Chicago, they were in the same city as each other, at the same time; they lived near each other; they had the same political interests; their circles of friends and associates intersected.
Or was it not a coincidence at all?
http://www.zombietime.com/obama_and_the_weather_underground/
Read the whole article. It’s very interesting.