。
She introduced herself to the people who were there.
Literal
She [topic-は] there [at-に] exist people [to-に] self-introduced.
そこにいる人たち is a relative clause modifying 人たち ('people'): 'the people who are there'. Japanese builds relative clauses by simply placing the modifying clause directly before the noun, with no relative pronoun like English 'who' or 'that'. The verb いる (animate existence) is used here because 人 are animate; the inanimate counterpart ある would be wrong. The second に marks the recipient of 自己紹介する ('introduce oneself'). 自己紹介 is one of the canonical ritualized speech acts in Japanese social life — newcomers at a school, workplace, or club typically deliver a short self-introduction the first time they meet a group.