Hera,
I totally agree with your comment, and can let you know what I have seen from the many guilds I have been in that have websites.
1. The last guild I was in had the same exact problem. Everyone was complaining that they were not getting into runs, even though it was stated on the calendar that to sign up you had to go to the website. The GM also tried to get people to put their schedule down to no avail.
2. The guild Gorg and I were both officers had people get into arguments in the game on why people should go to the website to register. This lead to several toons just leaving the guild, since all they really want to do is just play the game.
3. People just do not want to go to a website, and want to escape the everyday computer use. I agree that it really is not that hard to get to, but some are just not going to do it and as much as I hate it we have to respect their wishes.
I know this probably does not help, but it is a common issue to most every guild out there. I am not sure how you make everyone happy, except to make it known to all that you will put out a global invite to anyone that is gear ready and the first ones to sign up get to go. You can also state like you have that you will try to balance out the raid, ie 2 melee, 3 ranged, etc.
Good luck with trying to get this going. If you need help let me know.