Grum will stretch his arms wide, dagger and shortsword also extended, and will quickly move through the room, waving them in thin air, to sweep the room for someone invisible (no idea what kind of check, if any, might be relevant for that!)

Following a safe distance behind Grum, Callie spares her companion a glance before making a beeline for the hidden path. After looking it over to make sure that it is indeed there, Callie calls out to the others, "Everyone, there's a secret door over here." When the others are ready, and assuming that nobody tells her not to, she will open the door with a hand resting on the hilt of her sword.

Can I make an investigation check too

Yes, It would be an investigation check for Grum and Garr to search the room.

22

Investigation: 15 

Callie pushes open the door, revealing a stairwell to the east.

While she does so, Grum waves his blade through the room as Garr follows behind and tries to notice anything out of the ordinary. Grum finishes his sweep, detecting nothing with his blades but as Grum turns around he notices a small kickup of dust leaving behind half a footprint. As he does so, the footprint shifts and a spiked club apparates out of nowhere, coming down towards Squelch.

Attack: 24 Damage: 18 + 8

Edit - Since Garr's initiative is higher you don't take the 2d6 damage on top, so ignore the second number. I wont remove it otherwise it might change the other dice results. Garr also gets to take a turn of combat before this bugbear takes his action.

Everyone is Surprised except for Garr as the figure attached to the weapon also appears out of nowhere-- a Bugbear with a dark orange hide and scars lashed across their arms, smaller than the others but still a burly creature and glaring with a sharp expression at Squelch. Droop, who had been following behind currently in the wizard's workshop passes out again upon seeing this unfold.

Everyone please roll Initiative!

Bugbear: 6

(Omg that critical...)

Garr Initiative: 5

Garr fires his crossbow with his action and bonus action, hoping to perhaps cut down the bugbear before his attack hits (as unlikely as it may be)

Attack: 12 Damage: 9 +1

Attack: 19 Damage: 5 +3

<Squelch> silently drops from the surprise attack.

Initiative: 11

Callie Initiative: 18

Grum Initiative : 13 

Malth initiative: 14

Initiative: 13

<Start of Combat>

As you all draw your weapons and look upon this new enemy, a subtle wave of pink energy floats through the air from the back of the room and swirls around Garr and Callie, nearly invisible. I need those two to make a Wisdom saving throw. Another foe makes an appearance, a short, dark-bearded human male in red mage robes. He wears a princely mantle of ermine and is holding a beautiful glass staff in front of him as he casts this spell.

Initiative order:

• Glasstaff
• Callie
• Malthaza'ar/Garr
• Grum/Brandor
• Squelch
• Bugbear

It is now Callie's turn after the WIS save.

Callie WIS save: 13

Garr WIS save: 21

You both feel some force drawing you to this mage, as if alluring you towards him. You both snap out of it however, now realizing that the mage was attempting to charm both of you.

"Shit." The mage mutters as he grips harder onto his staff.

Callie you can continue with your turn.

On seeing the glass staff and realizing that the caster just tried to do... something, Callie immediately draws her sword and lunges at Glasstaff with a panicked expression on her face.

Attack: 23 Damage: 5

Malth turns up her palm and draws it to her waist in a threatening gesture.

She casts Magic Missile at the mage.

Damage rolls: 9