This year its at Radio City so its the largest venue they have ever had, so getting in should not be a problem.
Here is the deal. There is a line every year, and they have turned people away. But like I said I do not see eople getting tuned away at Radio City. But the draft is like a 8 hour event, lots of people show up for their team to pick and than leave, so they will keep letting people in. Throughout he day. Also you can wait by the exits and as people leave they will generally give you there tickets so you can go in and watch your team pick.
Its such a long day, and really when you are there it is kind of exciting geting caught up in the many different fan bases there booing every pick that is not their teams, and getting into it with each other. Since the Pats pick late every year, I usually go to one of the many bars in the area with a group of friends, watch the draft, drink, eat lots of food that is bad fo you. Than head over at like the 15th pick, (about 2 hours after the draft starts) there is no line by than, you can get right in, and than hang out for another 2 hours in the place. The draft is cool because the ESPN guys will banter wih the crowd, some of the players will greet you, the ribbings between fan bases, you get all the draft junk and players books that they hand out .
If you have never been I suggest you go once, yes the down time between picks can be boring, and when the Pats 1st pick is 2 1/2 hours after the start of the draft it can get tedious sitting there, but it is still a fun experiance.
And if you stay for the 2nd, 3rd or 4th rd you stay with the true die hard fans. but its smaller group and you get ito a very social with everyone who is left (usually only a few hundred people at this point).
And if you show up for day 2, you are no joke. I think ESPN lets you on the telecast at that point