plurals - Pluralization in Japanese: usage of -たち and -ら

I know that -たち and -ら pluralize the nouns they come after (or indicate a group that the noun is part of), but most of the time the plural in Japanese is implicit. When is it appropriate or necessary to use -たち or -ら?

(Bonus question: is there any difference except formality between -たち and -ら?)