If you don't have a scheduled time slot, show up before the doors open and wait in line.
Example, I went to RTAG last year and United did not have scheduled meet and greets, just FCFS and the first day the line ended up super long, so the next morning I was in front of the convention center 30 mins before it opened and went directly to the United area when I got in. Others were doing the same thing, but then I just had to wait for about 20 people in front of me for my turn.