The group of bizarrely dressed youths are taking over the cafeteria,
The writer has used a plural verb because of the proximity of youths, but the subject of the sentence is group. The sentence should read:
The group of bizarrely dressed youths is taking over the cafeteria.
If there are two subjects joined by and, use a plural verb:
My mother and father are coming to visit.
If the subjects are joined by or, the verb must agree with the nearest subject:
Either Danny or Sandy is handling it.
Either Sherlock Holmes or the Hardy Boys are capable of solving this crime.
Collective nouns such as family take singular verbs when the sentence deals with the group as a whole:
The Griswold family is going on vacation this year.
If the sentence deals with the family as individuals, then a plural form is used:
The Griswold family are going to fight all the way through their vacation.
Linking verbs in subjective completions agree with the subject, not the completion:
My favourite thing to buy is compact discs.
Compact discs are my favourite thing to buy.