Ultimately, you'll probably have to come up with your own solutions, but I think the main issue is to pick which parts to order, as the number is usually limited and your LUG members all have different views on what to pick.
Here are a few line of thoughts:
Have each member pick one favourite part, or more (so you can attribute the second one if there's a duplicate or something). This will probably be the worst solution, since you'll end up with a set of parts which may be totally unrelated to each other, and in which most members don't recognize their choices.
Do the same with more parts, 5 or 10, then try to reduce the list somehow to what LEGO is willing to accept (asking the members to vote on parts, maybe?). Can be a cumbersome and lengthy process, but at least your LUG members will feel more like they have a say in the process.
Have the persons in charge of the LUG pick the parts themselves. This can lead to a good cohesive set of parts which go well together, but you'll risk that some members don't like your choice. For example, you may want to pick all 1xn tan bricks and plates, all 1xn dark red bricks and plates, a few door/windows and roof tiles. Great for house builders, less great for Technic nuts.
Try to combine the different solutions. Maybe you have members in the club with a different status (we do have regular and "effective" members, for instance, for people who choose to be more involved in the club); or maybe you can attribute points for event participation (we did that once to distribute some free loot we had gotten from somewhere, and it proved to be an interesting system), so that people who are more involved in the club get more points. Then maybe more involved people can pick more parts, or participate ina global discussion process.
Rotate themes. This time, we'll do houses, next time, Technic parts. Promise, honest. Cross my heart.
Once you have picked up parts, you'll have to pick quantities. There you'll also need to ask members for their input, either committed or not. Our club always buys a bit more on its own money, which members can buy directly afterwards. Handy when the club committee picks up the parts themselves, but your treasury needs to be able to cope with that.
As for the organizational side of things, as you say, excel files may be very cumbersome; but I'd advise to stick with them. You may be tempted to develop a custom web site, for example, but keep in mind it will require maintenance, and that usually means someone has to be devoted to it, and be able to work on it fast enough. If you have such a person, great. If not, don't bother too much.