For the first case, I'd just ask, "Do you need help?" The thing is, you've said some of them might not have company email yet, so they should get a pass. Or they may be looking for a different company altogether and accidentally gotten off on the wrong floor. Or the security guard downstairs was swamped with people asking questions and didn't give them all the information they needed. Or it was noisy, with lots of people talking and they thought they heard what the person security desk said, only to realize when they got to your floor that they didn't.
Because you can't know the circumstances that led to them standing there not knowing what to do, I think it is best to err on the side of them not having all the information they need. Then you can direct them to the security desk, or elsewhere, as appropriate. If this happens often, like more than once a month, I'd report it upstream, because it could be a problem with the system, not the individual employees. Maybe the emails aren't being sent out in a timely manner, maybe one of the security guards isn't giving out the right information, etc.
For the second, it depends. I was in a similar situation once, where my desk was closest to the door. But co-workers at two other desks could also hear if someone knocked, so we sort of took turns letting people in. However, this didn't happen very often--maybe once or twice a week at most.
But if you are getting up from your desk daily to let people in, then I think that's too much, in an office where people know they need to carry their badge. If it's one person repeatedly, I'd speak to that person and let them know that they are interrupting my work daily, and that it needs to stop. Or eventually, I'll have to report them to their supervisor.
If it is many people over the course of a week, but not one person with a ton of repeat offenses, I'd go to a supervisor on the floor and ask for help, because clearly, people need to be retrained. Each employee probably sees it as "just a minute to let me in," and not the 5 minutes it takes to regain your track of thought, or the fact that the OP might be getting up from her desk 7 or 8 times a day.
For the tailgating, it depends on company policy. Where I work, we can tailgate, but we must still swipe our badges to indicate we've entered a section of the building. They keep track of employees because we have a lot of confidential info in the place and there's a risk of someone stealing it and selling it. If you aren't allowed in an area, the beep the system makes changes and the person in front of you is not supposed to let you in. They are getting stricter and stricter about this.
Now it is not uncommon to come through the door onto your floor in the morning and find a senior level employee just casually hanging out near the door--they are there to check to make sure everyone is swiping their badge. They don't get upset if you hold the door for someone, but they do get upset if you don't swipe your badge. (At times, we have a lot of temps, over 250, in our building and there's no way to learn their faces, but forcing everyone to let the doors close completely would mean that it would take much longer for everyone to enter the building and get to where they are working--and we'd have people backing up on the stairways, which isn't safe. It's a compromise.)
I'd follow company policy to the letter, and if anyone complained, I'd point out the policy. This is what I do and while a few fellow employees have complained to me about it, my boss has commented in my review that I take company policy seriously. This has led to at least one promotion, because I'm trusted with the confidential stuff.