You can also go into BWL solo iirc. At least, if you are in a non-raid party it'll give you each solo instances of MC and BWL. It might be a 40-man raid thing.

For other areas, if your computer can temporarily handle it, you can launch another instance of the wow game, and log in to a never-expiring trial account. From your real account, group invite any of your trial characters that are level 10 or above. Tada, raid group, and no having to bother friends/guildies/beg in trade. After the raid group is formed, you can exit the game on the trial account (but stay in a group) to get full performance from your computer back.

You can also get in if you have done a random heroic, and didn't leave the party. If everyone else leaves, you can still use that 'party' that you are in to get into the raids without requiring another person.

@mycroft
I've been doing this - works out pretty good - just set EVERY setting to minimal - and obviously no addons except an auto-accept addon of your choice (just easier). Getting a trial toon to lvl 10 takes less than an hour and makes life easier...

My trick is to /who a capital city and find a lvl 1 (those are usually bank alts since they haven't leveled) and invite them. Then no one is really harmed.

Indeed...your guildie doesnt HAVE to stop his questing or anything. If I wanna enter an old raid, I usually ask someone in my guild to log an alt for 5 secs, invite that alt, make the group into a raid, then the guildie logs back to his main or whatever char he or she wants to use, and I head to the raid instance.
So that person doesnt have to stay online on that char, and usually, everyone has at least one alt they can "borrow" you for that.

Also you can do a cross-realm group with Real ID as long as it is the same faction. Just group, change to raid and you are good to go.

Wait, what!? You mean the instance doesn't try to kick you out or anything? How long has this been a thing? :O

I was able to solo within Karazhan at one point. I had downed two bosses and then the person I asked to join me for the raid while not inside with me, was in town dropped group. I was expecting to get the boot from the instance but it never came. I ended up running through the whole thing without any error, could someone explain that for me? I don't understand it myself.

When I want to hit up Karazhan I'll log to an alt, invite a guildie, make them leader and hop to the toon that I'm running it on. Then they can drop group and I'm off to the races.

Yes, while you can get into BWL by youself, I wish you luck if you attempt Razorgore (and you have to attempt razorgore) solo. He's difficult enough to 2-man.

@murmaiderxx: This is how it has worked for quite a while. The check to see if you are in a raid party is done when you go through the entrance portal. As soon as you enter, the other player is free to leave the party and go about his business. You can continue inside the raid instance just so long as you don't leave for any reason, including dying.

An Auto-Invite addon is a very good idea, I even modified mine so that I could send the other character a whisper, and it would shut down the other client.
Speaking of the client, I have a shortcut on my desktop that links to:
"D:\World of Warcraft\WoW.exe" -nosound -windowed -800x600
You obviously have to adjust the path. I tried resolutions below 800, but it rarely worked then.

@murmaiderxx/twbrienn: You don't need the raid group once you're inside the raid (unless you wipe and have to run back inside) and you don't get booted from the instance if you are the leader when the group is disbanded. Kick the other character or have them leave, and you don't have any problems.
You can even stay partied with the other character, to chat while you're both doing your own things, since after you entered the raid, you can right-click your portrait and select "Convert To Party" and voila, your party is a non-raid again :)

I'm fact, you don't even need to level your trial account char to 10 in order to use them to form a raid. There's a command to enable low level raids (the exactly command escapes me, though you should be able to find it fairly fast with google). I've been using a level one draenei with the endearing name of "raidmule" to solo low level raids. It's deluxe.

But i STILL can't get prince to cough up the Sunfury Bow of the Phoenix for my hunter. That guy is a jerk.

I found http://wowprogramming.com/docs/api/SetAllowLowLevelRaid