Tell people in advance that there's a Less Wrong sign and that they don't need to worry too much about finding the right people is a great suggestion. I added it to the meetup guide (the "Making the announcement" section):

Wherever it is that you are meeting, you’ll want to bring a sign from which people can recognize the right group of people. Mention the fact that you’ll have a sign in the meetup notice, or offer some other easy visual cue for recognizing the group! This way, people don’t need to be nervous about whether they’ll find you. This is especially recommended if you’re meeting in some large location.