Is there a committee that is maintaining the language e.g. accepting/rejecting new words? And what are the processes/policies there?
There is Akademio de Esperanto who can "officialize" new roots. But they see themselves as descriptive and not prescriptive. So if a new word root makes it in to everyday's usage of a significant part of the Esperanto speakers they might eventually officialize it.
But this officializing has closed to zero relevance in practice. Language is, what people speak, not what a committee decides.
The Akademio de Esperanto is a language academy consisting of well-regarded Esperanto scholars and writers that oversees the evolution of Esperanto based on its foundational principles. It is the successor of the Lingva Komitato founded by Zamenhof.
The Akademio de Esperanto does not actively propose new words. Instead, it lets the Esperanto community try out various proposals for new words, and only sanctions them by "officializing" them once they are widely used in a coherent way and are not in conflict with the foundational principles of Esperanto. The Akademio de Esperanto also occasionally makes decision on grammatical questions, and on other questions of language use, e.g. most recently on the question of when and how to Esperantize proper names.
Additionally to the official decisions by the whole Akademio de Esperanto, its section Lingva Konsultejo responds to any questions posed to it about Esperanto grammar and vocabulary. Most of these responses can be read online.
Overall, the Akademio de Esperanto only plays a small role in the evolution of Esperanto. Most of the evolution happens spontaneously, based on the conscious decisions as well as unconscious changes of habits among Esperanto speakers across the globe. However, the Akademio de Esperanto does have some influence. For example, its recommendation on country names caused the usage of such names to be more uniform.