In fact , the sentence is a complex sentences that is included two dependent sentences. Who is the start of the second sentence and is related to teenagers. please pay attention to following : A complex sentence is made up of an independent clause and one or more dependent clauses connected to it. A dependent clause is similar to an independent clause, or complete sentence, but it lacks one of the elements that would make it a complete sentence. Examples of dependent clauses include the following: because Mary and Samantha arrived at the bus station before noon while he waited at the train station after they left on the bus Dependent clauses such as those above cannot stand alone as a sentence, but they can be added to an independent clause to form a complex sentence. Dependent clauses begin with subordinating conjunctions. Below are some of the most common subordinating conjunctions: after although as because before even though if since though unless until which who when whenever whereas wherever while A complex sentence joins an independent clause with one or more dependent clauses. The dependent clauses can go first in the sentence, followed by the independent clause, as in the following: http://www2.ivcc.edu/rambo/eng1001/sentences.htm

In this case, we understand that "who" refers to many people, and not just one.

Because teenagers is a people , so,use who

My 2 cents; "Many teenagers who dream to be a famous singer join in the singing" is a difficult sentence to parse, Is it an "imperative" sentence? Then "Any teenagers, who dream to be a famous singer, join in the singing. Imperative: To command some one or group of people WHAT to do. If it is ti be a statement of fact: Many teenagers who DREAMED to be a famous singer JOINED in the singing. Past Tense STATEMENT of what happen. Or similarly, by changing the tenses to be future tense.

Everyone thanks a lot.