Well, we're talking about large buildings with thousands of people going in and out every day. I suspect it had a large administrative and maintenance staff. The larger the organization, the more likely one will see new and unfamiliar workers, and it might be easier for someone to infiltrate. When I visited the WTC, I don't recall that security was all that tight.
Here's a possible scenario: Government agents could approach the building management and say they suspect the drug cartels have a money laundering operation, and one of their offices is located in the WTC (disguised as an apparently "legitimate" company). So, they could say they need to put in some undercover operatives posing as maintenance personnel, as well as install some "surveillance equipment." Now, of course, it's all top secret and hush-hush, so as far as the rest of the staff and tenants are concerned, they're just some ordinary work crew which has been given the okay by the boss (who himself would not even know what was really going on).
As has been pointed out by many, there aren't that many people who actually have expertise or professional knowledge about engineering or architecture or even really understand how buildings are blown up. If this is true, then very few people would have been able to recognize or notice someone engaging in demolition work, and if their top boss tells them "don't worry about it, they know what they're doing," they would probably just leave it alone.