It's fine to go with them to try on/pay for their dresses, and it's very nice to go out to eat afterward.
However, I think you are going to run into a lot of problems and hurt feelings if you start telling people what you "expect" of them. All a bridesmaid or groomsman is obligated to do for your wedding is come to the ceremony on time and in the proper attire, stand quietly for the ceremony (or sit), and smile for the photos. That is all they HAVE to do. Most people will probably do more, as long as they have the time/money/desire to do so, but you cannot REQUIRE them to do more without coming across as a giant bridezilla.
If you want some help with planning or tasks (maybe putting together invites, shopping, someone to accompany you for a hair or makeup trial), you can definitely ask them politely if they would be able to help you out. It's fine to ask for help, but women are bridezillas when they EXPECT or DEMAND it. It is not a bridesmaid's job to help plan your wedding, or be at the bride's beck and call. If she can help out, great. If not, deal with it.
No, you don't need "favors" for this get-together. If you want to pick up the tab for your meal afterward, that'd be nice. If everyone is paying their own way, be sure to choose a place that everyone can afford, and don't make people go. You cannot demand anything that you do not plan to pay for in a wedding ... so that means you cannot *demand* that they get their hair and makeup professionally done. You can say, "I found a stylist to do my hair and she would charge bridesmaids $50 each. Let me know if you want to use her." You are not obligated to pay in this case because you would be giving them the option to use someone else, or do it themselves. If you tell them that they HAVE to use your stylist, or HAVE to have it professionally done (even if they can pick the stylist and/or their own style), then you need to pay. Period.
And under no circumstances can you tell them to plan you a shower or bachelorette party. You cannot delegate the hostessing job to anyone, and you cannot tell them that you expect to be thrown these parties. You need to keep your mouth shut about them, and deal with whatever you may or may not receive. Brides are not owed these parties. You can give input on dates, the guestlist and activities if you are asked about them, but other than that do not say a word about them, and under no circumstances should you have a hand in planning them. They are parties that would be thrown in your honor, for you, so it would be horribly rude to get involved or outright ask for them.
If you don't like being the center of attention, then don't have a "meeting." Just go with them to try things on, then go out and enjoy your lunch/dinner afterward. Don't turn the day into a wedding meeting where you start giving out jobs and tasks or telling them that you expect certain things from them, because that's a surefire way for people to not want to be involved. Politely ask for help if you need it, respect people's time, be nice to your friends and family, don't center the next 5 months around your wedding, and you will be fine.