As group leader, you change the roles of others by right-clicking on their name in the group window, and selecting role, then whatever role you want them set as. If they are already set to a role, they will have an icon by their name in the group window indicating which role(s) they are set for, and the right-click menu will have 1 or more roles checked.
To change your own role as group leader, you right click on your personal window (usually your name in that window, but some custom UIs have different hot spots for the right-click menu).
Group Main Assist role, along with the other roles, are now available without any Leadership AA and any number of people. Group Main Assist, prior to mercs, required 3 people at least rank 1 of the Leadership AA (LAA). As Main Assist, you will see a special target ring on whatever your target is. Solo or molo, it may not be as obvious, since you will also see your normal con colored target ring, but when someone else is set as MA, it will be obvious when they have something targetted that you do not. The MA role also allows using the
/assist group command to assist whoever is marked as MA in the group.
Setting yourself as puller, will make your merc stop where ever they are if no one else is in the group, or follow another person if more than you. Setting them to passive, assuming you're not set as puller, will let them continue to follow you until you get a mob to where you want. You can also turn off auto-assist, and use the Assist button on the merc window to tell them when / who to attack, but if they're set as Main Tank and Aggressive mode, they decide to assist earlier (I can't remember for sure, as I rarely use Manual Assist on a tank merc).
With Auto Assist on, and MA role set, your mercs will beat primarily on the MA's current target.
Setting the MT role helps tank mercs alot in that they generate more hate when marked as MT and attempt to get aggro on all the attacking mobs.
The set active button just sets the merc to the currently selected aggression role. Just selecting it does not set it. Also, holding the left mouse button down on the set hotkey button for a few seconds will create a hotkey on your cursor for setting the selected mode. Set owner button is for giving your merc away to your target.
Discs are disciplines (Alt+C window). Somewhat like spells for priests, casters and hybrids, but use up endurance. Many of the most useful have long reuse timers. The ones you want to watch out for are the best defensive ones, which start as
Defensive Disciple, and includes
Stonewall Discipline and
Final Stand.
Furious Discipline, a riposte line of discs, are also good to keep an eye on, as that may be a time to run. The merc versions of these discs may have a slightly different name, as they're available to mercs earlier than real Warrior's, but the names are similar enough to know what they mean, and the emotes are similar enough as well.
As a Pal, you get a few disciplines, such as
Deflection Discipline, but much fewer than a true (non-casting) melee class.
Yther Ore.