Since this is a large stationary object, you hit. The attack was more or less to see whether your foot lands solidly. Roll for damage (1+strength mod)

That’d be a 4, then. Can I kick it a few times?

You thrust your foot into the door, causing a dull thud. You turn to find no apparent damage, but it is stone so it will likely take a few hits.

You can wail on it as much as you'd like, and that goes for anyone else. I will say that piercing and slashing damage would not go as far as bludgeoning for obvious reasons. Or if anyone feels like attempting another method, then you could pitch that between hits.

I guess i can’t  let this door fly open with Thaumaturgy?

But Happy could try to help with an Eldritch Blast (force damage)

No, Thaumaturgy won't have enough force behind it to do anything. Eldritch Blast will more than likely have an effect though. Roll for attack and damage, if that's what you want to try.

Do we really need to roll if we’re just going to keep whacking it?

Not sure what was going through my head. No, you don't need to roll to attack. With a combination of kicks and Eldritch Blasts you both get the door to the point where it is heavily cracked, and on the verge of breaking. With one final combo, you both send a cascade of rubble tumbling in. The stone sounds as if it begins tumbling down stairs. As you bring your head closer to check it out, the smell of rot fills your nostrils causing you all to instictively recoil and cover your nose.

Regaining your composure, you look through the opening and indeed see a flight of stairs going down about 15 ft.

‘’Well. That’s definitely a better way to open the door.’’ Sieghart uses his cloak to cover up his mouth and nose to protect himself from the foul stench of rot and decay. With his other hand he brings the torch forward as he begins his descent in the stairs.

You descend down the stone stairs. A little bit of the stench makes its way beyond your cloak, but it's still better than going without. Reaching the last step you find yourself in ractangular room, 20 ft wide and 30 ft long. In the dim reaches of your torch you can see a set of double wooden doors at the far end, a passage barred by a portcullis 20 ft along the right wall, and an open passage directly opposite where the ambient green light, that you all have grown used to, is more intense. It looks as if the light itself is curling around the doorway and trying to escape. The room itself is devoid of furnishishings or decor.

Edit:

(We'll continue on with Arlo moving toward the green light.)

Arlo, as you make your way across the room and look for the source of light, you are startled by the sudden crash and gargled shouts of a pair of creatures on the other side of the portcullis. You turn to see two gaunt, hairless, ashen skinned humanoids. Both are barring their razor-sharp teeth, and thrashing around with their jagged claws attempting to bust through the barrier, assuming with the intention of making you their next meal.

Sieghart signals to the rest of the group that the coast seems clear. He starts heading towards where the green light was emitting in the left room.

(You will be included with what Arlo experiences.)

Arlo and Sieghart are at the green lit passage and are distracted by the sudden appearance of two viscious humanoids. Happy, Lucas, and Catherine (now controlled by me) are at the base of the stairs.

What do you all do?

Can I tell what those things are? And does it look like just a cell beyond the portcullis?

Roll an Intelligence check to see if you can recall what they are or anything about them.

It appears to be an empty room from what you can tell, though you cannot see the full size of it.

(Can I make the roll too? Just disregard my roll if I can’t)

Intelligence: 17

Arlo Int 18

(I have darkvision - 60ft , can I scan the floor and walls for levers or trap buttons?)

Lucas enters the room behind them, unfazed by the ghastly creatures behind the portcullis. He scans the floor and the walls around the portcullis, looking for anything that might control the gate. 

"Let's tread carefully, we don't want to let those creatures out of their cage prematurely"

Investigation: 4

(Yes, you both can have rolls. Mental stuff I'm ok with multiple people doing simultaneously, because no one else knows what you're thinking. Physical stuff is where I put a stop to everyone jumping in on a skill dog pile.)

Arlo:

 Sieghart:

 Lucas:

Happy asks: “Did you find something interesting? What are those creatures?”

Sieghart recognized the creatures. It was a good thing that the portcullis prevented them from getting closer.

"Ghouls...These things mean trouble. Don’t get too close. It’s believed that their claws can infect you with toxins that restrain your movement."

"Hey Sieghart, can you shine your torch up here?"

Lucas points to what looks like a gem embedded in the center of the ceiling. 

Investigation: 11