I have read reports that he attended the University at the time, and that he was part of the group -- but according to other sources, he is not the one in the picture taken. See my post below for specific details.
Edited to add, by "part of the group" I mean, it was never specifically determined he was part of the "group" that actually took hostages, but that he was only possibly associated with the larger movement. I'll try to find a link to what I'm trying to say, here, for clarification.
Edited again to add, This is an example of some of the reports that I had read that claimed he was part of the larger group:
~snip~
Former Iranian president Abholhassan Bani-Sadr, who lives in exile outside Paris, told The Associated Press on Friday that Ahmadinejad "wasn't among the decision-makers but he was among those inside the Embassy."
Bani-Sadr said Ahmadinejad was responsible for briefing Supreme Leader Ayatollah Ruhollah Khomeini on the hostage situation.
"One of his roles ... was to inform Mr. Khomeini of what was happening at the Embassy," Bani-Sadr said in a telephone interview.
Hajjarian denied those allegations as well.
Bani-Sadr said the new Iranian president was initially opposed to the hostage-taking but, according to his information, changed his mind once Khomeini gave his agreement.
~snip~
http://66.102.7.104/search?q=cache:uEC-a36EDSYJ:www.heraldonline.com/24hour/world/story/2529994p-10909047c.html+Mahmoud+Ahmadinejad+Taqi+Mohammadi+&hl=en&gl=us&ct=clnk&cd=21But even this information is not well documented and remains questionable.
All in all, I think we should take all of these accusations with a grain of salt, considering what occurred pre-Iraq. Here's another excerpt that highlights much of what I had read about this situation:
The supposed AP photo that surfaced after Ahmadinejad's election victory was shopped around by Iran Focus, an anti-regime group with ties to the US Alliance for Democratic Iran, and the neo-con Foundation for the Defense of Democracies (FDD). The still photo has now been supplemented by color video footage showing the Ahmadinejad look-alike Mohammadi. Some of the ex-U.S. hostages who now claim, over 25 years later, that Ahmadinejad was one of their captors, have strong connections to GOP and conservative politics.
FDD's principal players include Newt Gingrich, James Woolsey (the friend and supporter of Ahmad Chalabi and his Iraqi National Congress), Gary Bauer, Richard Perle, Cliff May, Zell Miller, Jeane Kirkpatrick, and Tucker Carlson's father, Richard Carlson.
Perle, in turn, is a primary supporter of the Iranian exile People's Mujaheddin (Mujaheddin-e-Khalq/MEK), a group designated a terrorist organization by the U.S. State Department and which operates unhindered from U.S.-occupied Iraq. The MEK and its affiliated arm, the National Council of Resistance, have now surface as the likely source of another story claiming that Ahmadinejad was an assassin sent to Vienna in 1989 to kill Kurdish opposition leader Abdul Rahman Ghassemlou.
This story was first shopped to a Prague paper, which claimed its source was Hossein Yazdan Panah, and Iranian Kurdish activist based in U.S.-occupied Iraq. The story was then was picked up by Austrian Green Party official Peter Pilz, who claims his source for the story is a mysterious "Witness D," an Iranian journalist based in Paris, a city known as a major operational base for the MEK. (This is very reminiscent of Ahmad Chalabi's "Curveball" source, the congenital liar and drunkard who claimed first hand knowledge of Saddam Hussein's mobile biological weapons labs, charges later proven to be fraudulent).
Ahmad Chalabi, now Iraq's Oil Minister, was a math professor at the American University in Beirut in the 1970s and, according to a longtime ABC correspondent in the Middle East, was known as SAVAK's man in Beirut. (SAVAK was the Shah of Iran's feared intelligence service some of whose members were trained by the U.S. and Israel).
http://66.102.7.104/search?q=cache:p57W8KsZ05MJ:xiaodongpeople.blogspot.com/2005/07/diplomatic-cables.html+Taqi+Mohammadi+Mahmoud+Ahmadinejad&hl=en&gl=us&ct=clnk&cd=10