Group Manager is run and updated by the Essentials Crew! here's the link:
dev.bukkit.org/server-mods/essentials/
Here's where you'll find the link:

Once you download that, you should receive a .jar file. Now, open up your server folder (I am showing pictures of my host, volt-host.com. Your files should be named the same, mine will just look prettier.)
So, open up your plugins folder:

Once your in there, copy and paste your .jar file into your folder. Or look for the upload button if you use a online host, like me. When you are done, it should look like this:

Then, reload your server (If you dont have bukkit, restart it. The command in bukkit is /reload and the alias is /rl.)
Now, you should have this:

Along with this:

If you have both of those, you have successfully installed Group Manager. Lets start making groups.

Group Manager's command have simple meanings. Every command will start with /man for manager, basically the basic command of Group Manager. the letter following that, either u or g, stand for user and group. So, this means:
/manu (Manager, user)
then, after that is what you want to do. So if I did add, after /manu, we get this:
/manuadd (manager, user, add.)
This tells the manager to add the user to a group.
So we would use the command such as:
/manuadd derp123 Moderator
Time to show you the commands.

The basic command for group manager is /manuadd. This lets you change which group a player is in. So,
/manuadd derpyderp moderator would add derpyderp to the rank Moderator.
"OK, cool. I can add players to the groups I don't have."
Actually, Group Manager add default ranks. One of the only commands that do not start with man, is /listgroups.
You should have: Owner, Admin, Moderator, Default, Builder, then alot of ones that are like G:(something here) those g:(somethinghere)'s are global groups. Basically, if you have a second creative world, that group will transfer over. These can only be used in inheritances, which I will show you in a minute.
The next important command is /mangadd this just creates a group. So, /mangadd Co-Owner would create the group Co-Owner. There really isnt much else to it.
"COOL! I got ranks that dont have titles or anything!"
Yup, on to our next important command. The variable command.
/mangaddv (stands for variable) (prefix/suffix)
*Notice, anything in '()' that are in a command, are not important to the actual command, its just nifty info.*
So, if we did /mangaddv Owner prefix &0&o[&4&oOwner&0] &f&o will give my personally favorite title for owner:

*Yes, I do own a server. It is currently PRIVATE while I build the map and do configurations.*
Now, for all you people who know latin out there, prefix means before the word/phrase, and suffix means after. So yes, you can add suffixes, but they do not look very good and I do not advise using them.
So lets make a list of what we know so far: (except for me, I know EVERYTHING, MUAHAHAHA)
Add players to groups,
Add groups,
Add variables.

Now earlier, I talked about inheritances. When you inherit something from someone, you inherit whatever they assigned to you, typically their title and wealth. In Group Manager, its commands. So lets say I have two ranks, Owner and Admin (And I did not give Owner star perms [*Star perms are permissions to everything...]) And Admin has all essentials permission and owner has every permission except for essentials. If I typed this:
/mangaddi (I for inherit) Owner Admin, all the commands Admins have will be given to the Owner also. This also lets you use the /manpromote and /mandemote command. (just follows inheritances. So if it went Member>Mod>Admin>Owner, and those all inherit the prior, then you could just type /manpromote noob [who is a member] and they would be made a moderator)
"Cool, I can add groups, make titles, add inheritances, and move members. But my groups dont have any permissions!"
All you need to do is /mangaddp (for permission) . And your done.

**Just a quick comment, I kinda took it for granted that you would understand the differences between U and G, del and add. If you did /manuaddp it would add a permission to a player, and then if you replace add with del, it will remove the permission/group/variable.**

These are just quick, easy nonessential commands that you may use:
/manglistp check if a group has a certain permission.
/manglistv " " variable
/mangcheckv(andp) checks if group has certain prefix.

These are all the important commands. Remember that you can switch out G for U in every command, and the same with add and del. You can now start your own permissions set up.

See, most people would just tell you the commands and then let you fail. But I won't :) If your completely new to this whole thing, heres a quick list of ranks that I would have along with prefixes. YW <3
Owner &0&o[&4&oOwner&0&o] &f&o
Co-Owner &0[&4Co&0-&4Owner&0] &f
HeadAdmin &4[HeadAdmin] &f
Admin &c[Admin] &f
TraineeAdmin &8[TraineeAdmin] &f
HeadMod &1[HeadMod] &f
Moderator &9[Moderator] &f
TraineeMod (I still have not found a very good name for this rank. Comment on any ideas?)
TraineeMod &b[TraineeMod] &f
Default &6[Default] &f

Every plugin will list their permission nodes. I promise. It will just be on the front page. Here is a example; in the ScheduledAnnouncer2 plugin (located here: ScheduledAnnouncer2)

(Sorry for the length, but most plugins will have 20+ permission nodes.)
See how the permissions are shown? And what I circled shows you where the rest of them are.
SO: /mangaddp Admin announce.moderate gives the admin rank announce.moderate.