The most reliable account comes from the historian Suetonius. An autopsy of Caesar was done and recorded, the first written autopsy [that we know of]. Caesar suffered twenty-three stab wounds, at least one was in the groin. In the melee of his attempts to defend himself, and then his resignation to his death, it was a chaotic assassination.

Although certain conspirators have nods to where they personally stabbed in some accounts, that seems to be conjecture. [Nicolaus of Damascus claimed Cassius, Decimus, and Brutus stabbed in certain spots but none were the groin]. There really isn't a way to say who stabbed where, unless they've got an undiscovered personal account of a conspirator somewhere and even then they might be blustering. The 'privilege' of saying you'd killed Caesar could be yours provided you stabbed him anywhere. Any first-person retelling might be exaggerated to what it actually was. "I stabbed Caesar in his heart!" sounds more brag-worthy than 'I stabbed him on the forearm'.

I can see why someone would dramatically claim Brutus had stabbed him in the groin. Groin can be lumped in as meaning the genital area. One of the reasons considered as a motivator for why Brutus joined the conspiracy against a man who'd been benevolent to him was that Brutus felt Caesar had dishonored his mother Servilla. He almost certainly did not believe Caesar was his father, whether or not it was true. Saying that Brutus stabbed Caesar in the groin gives that poetic justice feel. 'You've tainted the honor of my house with your manhood, and that is where I'll stab you'. It also gives an ironic nod to the 'theory' that Caesar was Brutus' father, as Caesar is stabbed in the organ which produced the seed of Brutus. But in actuality, would Brutus want to go around 'bragging' that he'd stabbed Caesar in the penis? Not likely. Were the adrenaline filled and frenzied conspirators taking note of who was stabbing where as Caesar struggled? Again, not likely. At least some of the twenty-three stabs were in incidental spots that came from trying to stab a man who wasn't standing still.

Supposedly when Caesar could no longer defend himself, he became resigned and threw his toga over his head which left his lower half better exposed. But then he drew his toga around his lower half as he collapsed, to spare himself a little dignity. In Plutarch's account, Caesar does this upon seeing Brutus, but that doesn't necessarily track with Suetonius' account of attacking a conspirator named Casca with a stylus.

Is it possible that Brutus accidentally or intentionally stabbed Caesar in the groin? Yes. Do we have any proof of that? No.

Edit: there's a nice account of Caesar's assassination in the podcast Giants of History in the Cleopatra section.

It's been awhile since I've heard it but an episode on it by the podcast History This Week was also good as I recall.