Let me start by saying I have not played D2 in a long time and if this has been nerfed/changed someone feel free to shoot it down.
When I played D2 I always played realms. Fastest way I ever found to level an entire account of characters to max level was with a lightning Sorc. You can google a basic light Sorc build or make your own. Very basic to make.
You can have any amount of alts in this. I always preferred to have a few extra accounts because I was selling the accounts.
What you need:
Lvl 99 Lightning Sorc with a very fast cast rate. Also you want a lot of Fast hit recovery in case a cow gets a hit in. You could possibly get away with a lower level character and then level her up as you level your alts. (Go on realms and find any number of games where they are leveling fast. Tag along or do your own thing to level.)
Alt account and players that want to level. (The more the merrier since more = better exp)
A basic understanding on how to play a nova sorc. (This comes as you level.)
As a bonus you can wear a lot of Magic Find gear as you run these. You get quite a lot of nice drops in hell cow level and a sorc can survive better than most classes with MF gear on.
How it all works:
Have the light sorc run all your alts through the game by teleporting to the act bosses, Then letting them portal down for the boss kill. Finish required quests in the same way making sure that the game is started by 1 of the low level characters. Open portals in a safe spot. Sorc can easily kill any and all bosses making this very fast. Get all your chars to where they are able to play hell games.
Open the portal to the cow level and let the Light sorc clear the entrance. Port all characters in and let them sit there. Run around gathering cows with the light sorc. Only the cow king will be unkillable but who cares. The exp is insane. The loot is not half bad either.
Gather cows. Static Field once or twice and spam Nova till dead, rinse repeat till you clear the level. Throw in a teleport to keep you alive if necessary. Mana shield, Lightning Storm etc. Cows die fast and exp is super fast. Make 100% certain to keep the cows away from your low level characters at the entrance. Keep cow king and his lightning away from them as well. Trap him in a corner and teleport out to lock him away.
When I was doing this I would fill accounts with lvl 70 characters and back then made a great deal of cash selling them before account selling was at all popular. While those days are gone this should still be a viable way to level. (I am not saying this is a way to make money any longer. I doubt that. Only that it is a great way to level mass amounts of characters to whatever level you want.)
Hopefully this all still works. If not someone will flame me and that will be that lol. GL and have fun.
P.S If there is a faster and easier way to level multiple people/accounts at once then feel free to list it here. Maybe I will pick D2 up again and come try it out.