You want to avoid a situation where people get in and out of the room during the presentation as they are not sure if they are at the correct room for the presentation. Having a slide with the title of your presentation shown on screen when people enter the room will help for the time before you really get started.
In addition to this you can also think about a professional looking entry sign on or next to the entry door(s) of your presentation location.
A laminated document on the door or a large banner, depending on the number of persons in your audience and the length of the presentation may be a good way of focussing the attention.
Use presentation title, timing and/or slogan to create interest and direction for your audience.