Returns the number of people and headers currently visible in the profession view of your guild window. This functions returns the number of people (including the headers that are also counted as one player each) which you can currently see in your guild profession window. If you select "Show offline Members" they will also be counted.
To make things clear a small example: let's assume you have all professions in your guild and the headers are all collapsed then this will return 11 (as of WoW 4.0). If you have 7 alchemists, 4 of them are online and you have not checked "Show Offline Members" then this will return 15. If you check the box to show all members then this will return 18.
See also Tradeskill functions, Guild functions.