Fellowship of the Ring  

A user under the nickname nexus1118 created a modification that adds 3 characters from the Lord of the Rings universe to Baldur's Gate.

Made the Fellowship of the Ring using a couple of mods: Party Limit Begone and Appearance Edit (including Origin Characters add-on because Gandalf was Gale). I had to boot 8 copies of the game window in potato mode while logged out of Steam to get the first 8, then swapped to single player (the mod allows 16 in SP) and recruited Gale and changed his appearance to be close to Gandalf. Current save has Shadowheart, Lae'zel, and Astarion in camp. The classes and appearances are as close as I could get to the movie with BG3's limited vanilla character creator. Guardians for them are also fluffy: Bilbo for the hobbits, Galadriel for Gimli, Arwen for Aragorn, Denethor for Boromir, Thranduil for Legolas, and some Valar looking high elf for Gandalf/Gale although I don't actually think his will ever show up.  The save starts at the ancient sigil right before the fight that introduces Wyll at the grove gate.  Difficulty is tactician because everything is easy with 9 characters.

Near as I can tell, you won't need to install any mods to run it properly because the party size limit is part of the save data and everything about the characters is vanilla.

Save location is here: C:\Users\<Your-user-folder-name-here>\AppData\Local\Larian Studios\Baldur's Gate 3\PlayerProfiles\Public\Savegames\Story
Unzip the folder then paste it in the story folder.

Just in case, Party Limit Begone
From that mod description: 

• Some ingame events are hardcoded for 4 party members and will glitch out. This is usually solved by either dismissing the additional members/players or by teleporting back and forth.

• • Camp sleeping bug: Use one of the awake companions and talk to Lae'zel or reduce your party size to 4 before resting.
  • Grymforge Map transition: Reduce your party size to 4 members before sailing with the boat. If you already used it and your characters are stuck, dismiss the ones that are with you and sail back and forth to unstuck the rest.  Note: After using the boat a timed quest is triggered and going to camp will progess it.

In my experience, you can ungroup then manually group 4 people into a separate group to do things like the Grymforge map transition.