What fills a niche in this party?

[Klorox]

I've been invited to join a 5e game. I know the players, not the characters.

All I know so far is the party is composed of:

wizard
bladesinger wizard
cleric
fighter
rogue

Other than the bladesinger, I have no idea what subclasses are chosen.

I'm playing a sorcerer in another game, so I don't want to duplicate that.

Ideas? Monk, barbarian, warlock are all free.

[KorvinStarmast]

I'd recommend monk. Been playing one and it's a nice compliment to both the fighter and the rogue.

[Specter]

Since all the classic roles are filled here (cleric-fighter-rogue-wizard), you should probably go for a 5th-man character with social skills. Bard comes to mind.

[Byke]

Every base is pretty much covered. Either a back up tank/melee DPS -->Pal, Barb or Monk or a ranged DPS specialist...BM/Rogue or Ranger/Rogue.

[Aett_Thorn]

OP, do you know what level you'll be starting at? You might be able to have some fun with multiclassing to fill some roles in here.

[Potato_Priest]

You can never go wrong with adding a paladin, so it would probably be a good choice without any other information. Team-wide saving throw buffs, good melee damage, and the second-best tanking class in the game makes it an excellent addition to anything but a stealth-heavy party, and even then you can use a dexadin.

It also has the advantage of being a Cha class, so you could do some minor face duties if no-one else is better.

[Waterdeep Merch]

I'd second paladin. Your team has a big melee focus, so I'd actually throw more eggs in that basket and make an alpha strike/lockdown melee crew out of it. Aura of protection will net you so much here, as will the ability to throw out big smites and dual tank. Your squishies will be very thankful. The backup healing power is icing on the cake.

[DarkKnightJin]

If the fighter is the melee kind, you could probably do with another frontliner for the Rogue to work off of. Barbarian or Paladin seems pretty good for something like that. Be in their face, make a big, obnoxious target out of yourself.. then hurt them when they stop paying attention to you.

You could even do a Barb/Rogue thing with a rapier.
Sure, it's not the yummy d12 of a Greataxe, but nothing says you need to use Dex for a Sneak Attack. Barb 5 or 6 can get you some pretty sweet things, not the least of which a second attack. Reckless Attack for at-will Advantage and therefore Sneak Attack. Advantage on Dex Saves, which then nullify damage if you clear them. If you go deep enough into Rogue for Evasion, anyway.
Oh, and you get a Bonus Action Dash to get across the battlefield with your boosted movement speed if you're not wearing Heavy armor.

...Crap, now I want to roll up a Barb/Rogue and have way too much fun with it..

[Temperjoke]

I'd consider a Blade Archfey Warlock. It gives you a little bit of everything, ranged magic, support spells and abilities, melee capabilities. You'll be able to adapt to what your group may need at a particular time.

[DarkKnightJin]

I'd consider a Blade Archfey Warlock. It gives you a little bit of everything, ranged magic, support spells and abilities, melee capabilities. You'll be able to adapt to what your group may need at a particular time.

Maybe go Ancients Pally/Fey Bladelock for a kickass Nature Knight that will ward their friends from evil magics. And visit the wrath of Mother Nature on their enemies.