Whenever the object of a sentence is a person (or similar), you must use the 'personal a'. This is also true for groups and plurals of people, such as muchas personas. (I add "or similar" because it can also apply to pets or things like that which we personify, though it wouldn't apply to general animals).
It also doesn't apply to standins for people that don't refer to a specific person.
Finally, we don't use it with tener when tener is in the sense of "Tengo un hijo.". But it would be used when tener is used in a more physical sense, as in physically having/holding rather than relationally having.
Disclaimer: this is me as a non-native drawing from memory. Google "Spanish personal a" for a more complete, more accurate description.
Yes, it's the "personal a" here. It is a preposition, and like all prepositions it goes in front of the grammatical unit it refers to, which includes the noun and all associated adjectives and articles. Since we're talking about "many European people" here, the words muchas and europeas are both associated with the noun personas, so they form a unit and can't be broken apart by the preposition. It's like in English, where you say "I learnt about many European people" instead of "I learnt many European about people."
If you said "Yo conozco mucho a personas europeas", the mucho would be an adverb modifying conocer, intensifying its meaning. It would mean something like "I know European people a lot."