This is actually a topic of some dispute, hence the discordance in your sources.
At the root of the debate are the plebs. One view, held since classical times, is that plebeians always meant all the common, free Romans, and thus part of the curiate assemblies from the beginning. The other view, advanced by the 19th century German historian Barthold Georg Niebuhr, argues that the plebeians originated as foreigners in Rome, and by extension were therefore excluded from Roman socio-political institutions such as the curiae.
Now, ancient sources do indicate that plebeians were, in fact, represented in the curiae even under the kings. Dionysius writes, of the election of the legendary Roman king Tullius, that:
The people being assembled on the day appointed, he called the curiae, and took the votes of each curia, one by one : And, being by all the curiae judged worthy of the royal dignity, he, then, accepted it from the plebeians, without shewing any regard to the senate, who refused to confirm the proceedings of the people according to their custom.
This clearly links together the curiae, the Roman people in general, and the plebians, who are contrasted against the patrician senate. More explicitly, Dionysius even reports that citizens of Alba Longa were ordered to be directly inducted into the curiae:
"I myself having assembled the Senate, and taken down their decree in writing, by which it is ordered . . . that your common people be Incorporated among the tribes, and curiae."
Of course, even the earliest known Roman annalists lived centuries after Rome had become a republic. Extant sources, such as Dionysius, dates to even more centuries later, in the Late Republic. It is probable that none of the materials they relied on could have dated to the Roman Kingdom. So the real question is whether Dionysius and ancient writers like him are correct about the regal period.
Niebuhr believed the ancients were mistaken. Moreover, he believed that by applying modern techniques, he could discern the real facts behind the primary sources. As such he argued that originally, the only Roman citizens were the patricians. In this view, plebians were barred from all political power, including participation in the exclusively patrician curiae.
Nibebuhr revolutionised his field, and his works were extremely influential on contemporary classicists; Leonhard Schmitz for instance writes that:
Our authorities, Livy and Dionysius, who were completely misguided by the meaning attached to the term plebs in their own days, conceived the plebeians to have been a low populace . . . this, with many other errors which had been established in Roman history, has been triumphantly and forever refuted by Niebuhr, whose great and peculiar merit it is, to have explained the true nature of the plebeian estate, and its relation to the patricians. The first plebeians, then, we repeat it, consisted of the conquered Albans and other Latin towns.
Schmitz, Leonhard. A History of Rome: From the Earliest Times to the Death of Commodus, AD 192. Harper & brothers, 1847.
Of course, "forever refuted" wasn't quite forever, and there has been more sceptical historians. For instance George Willis Botsford, a classics professor at Columbia University, adamantly disagreed with Niebuhr:
The position of Niebuhr has in the main proved untenable . . . though Cicero and the Augustan writers might misinterpret Fabius Pictor in minor details, it is inconceivable that they should fail to understand his presentation of so fundamental a subject as the character of the original populus or the composition of the earliest assembly.
Botsford, George Willis. The Roman assemblies: from their origin to the end of the Republic. The Lawbook Exchange, Ltd., 1909.
For what it's worth, some writers like Theodor Mommsen have argued a sort-of middle ground that even though the curiae were originally exclusively patrician, the plebians were admitted as early as the founding of the Republic. One could suppose this might have happened even earlier, during the regal period.
Either way, there is no extant evidence to definitively prove anything one way or the other.
