You have five major roles in a traditional everquest group:
1. Tank-This is the big guy who's keep the attention of the mobs and taking all the damage while the group dishes out the damage. This role is usually filled by a warrior, paladin, or shadowknight.
2. Healer-The one responsible for making sure that the group(especially the tank) is healed in a timely fashion so that people don't die. You'll usually see clerics, druids, and shaman as the group healer.
3. Crowd control(or CC)-These are the people who may seem like a luxury when everything is going well, but once something goes wrong, they usually end up being the group's savior. They're responsible for keeping adds(extra mobs that may be pulled along with the target, or any mobs that wander by the group) away from the group or incapacitated until the group is ready to engage. Classes usually used for this function are enchanters, bards and necromancers.
4. Slower/haster-The person with this job is required to slow the mob that the group if fighting as well as haste the melee fighters in the group. Classes that fill this role are enchanters, bards and shaman.
5. Damage dealers(or DPS-
Damage
Per
Second)-these are the ones who the group depends on to do the massive damage that kills the mob while the tank is keeping it's attention on him/her. Really any classes can fill this role, but typically groups look for wizards, rangers, rogues, berserkers or pet classes.
There are of course other jobs in the group(puller, porter, mana battery to name a few), but those 5 are usually considered the core parts of the group.
All that having been said, you'll find as you gain experience in the game, that sometimes the best groups are ones that don't fit the traditional roles of the group. Maybe you'll have five necromancers, or no crowd control, yet the group still does great. Or maybe you have a non traditional class doing one of the jobs, a ranger tanking maybe, and things still go well. It's more about working well as a team than it is about filling *required* group slots, as most classes can fill roles that they aren't traditionally considered suited for.
Your best bet is just play the game, keep track of the people you work well with, chalk it up to a learning experience the people you don't work well with, and eventually you'll find you have a core group of people that you can group with and be sucessfull.
As far as high level groups allowing lower level people to tag along, unless you know the group you're tagging along with very well, don't expect it to happen. Most people aren't going to be willing to group you unless you can contribute to the group. Otherwise you're just dead space, and there's a very finite limit on group numbers, so that makes group slots a premium.
Don't worry about making mistakes. They're going to happen, it's all part of the learning curve. Just when you do mess up, take a minute to think about what happened, what you might have done differently, use those errors as a learning experience. You'll learn your role in a group as you go(it won't always be the same role either, so be flexible), and don't be afraid to ask other players for advice. But above all, try things for yourself and find out what works for you.
And good luck!