According to Merriam-Webster.com and Dictionary.com, the word “myriad” can act as a noun or as an adjective.

“Myriad saw it.”

The first use of the source-word for “myriad” was in 1555 Greek.

The original Greek noun for “myriad” — “myrioi” — meant “ten thousand”.

Based on this, “Myriad saw it.” means “Ten thousand saw it.”

The noun “myriad” got generalized to “a great number”.[1]

Based on this, “Myriad saw it.” means “A great number saw it.”

“Myriad people saw it.”

The first known use of “myriad” as an adjective was in 1735, according to Merriam-Webster.com.

The primary definition of “myriad” as an adjective is “innumerable”.

Based on this, “Myriad people saw it.” means “Innumerable people saw it.”

“Myriad of people saw it.”

As noted, the definition of “myriad” as a noun got generalized to “a great number”.

Dictionary.com says that this is the primary definition for the noun; Merriam-Webster.com says that this is the secondary definition for the noun.

No matter which way you rank this definition of “myriad” as a noun, this definition tells us that “Myriad of people saw it.” means “A great number of people saw it.”

“A myriad of people saw it.”

This “a myriad of people or things” usage is problematic.

The word “myriad” in this usage is acting as a noun, but look at the two definitions for “myriad” as a noun:

• ten thousand
• a great number

Plugging in each definition, we get:

• “A ten thousand of people saw it.”
• “A a great number of people saw it.”

Each expression looks wrong.

Conclusion:
These three usages of “myriad” are clearly correct:

• “Myriad saw it.”
• “Myriad people saw it.”
• “Myriad of people saw it.”

This usage of “myriad” may be incorrect:

• “A myriad of people saw it.”

[1] This is equivalent to what happened to the verb “decimate”. It originally meant “to kill every tenth man”; its meaning got generalized into “to destroy in great number” instead.