When your audience joins a webcast it is good if there is something that they can listen to before the start of the webcast. By means of this they can test the setting of their audio and ensure every thing works.
With a smaller size of the audience you can have some small talk and as soon as a person joins you can welcome him / her. This is almost like you would do with a live presentation when the participants enter the room. You do have to be careful not the yet start talking about the content of the webcast presentation, otherwise for those who joined later, but still before the official starting time of the webcast will get the idea that they missed something and may get frustrated from the start.
With larger audiences you may want to play some entry music (can also work with smaller audiences). When using music you should think carefully about the atmosphere you want to create. Maybe something cheerful / energetic? Do also consider the copyright of the music that you intend to use.