Is the word demais the same in Brazilian and European Portuguese?

No

While the core grammatical functions of "demais" are the same in both dialects (acting as an adverb of intensity/excess or a pronoun meaning "the rest"), there is a significant difference in typical usage.

In Brazil, "demais" is frequently used as a colloquial adjective or interjection to mean "awesome," "great," or "too cool." In Portugal, using "demais" to describe something as "great" is much less common and can sound slightly out of place; a Portuguese person would instead use words like "fixe" or "espetacular." Additionally, while Brazilians use "demais" to mean "so much" (intensity), Portuguese speakers often prefer "muito" or "demasiado" to avoid the connotation of "excessive."

Brazilian Portuguese Examples

1. Você fala demais! (You talk too much!)
2. Esse filme é demais! (This movie is awesome!)
3. Eu gosto demais de viajar. (I like traveling so much.)
4. Não coma demais, senão você passará mal. (Don't eat too much, otherwise you will feel sick.)
5. Os demais convidados já chegaram. (The other guests have already arrived.)

European Portuguese Examples

1. Tu falas demasiado. (You talk too much.)
2. Este filme é muito fixe! (This movie is very cool!)
3. Gosto muito de viajar. (I like traveling a lot.)
4. Não comas demasiado, senão vais passar mal. (Don't eat too much, otherwise you will feel sick.)
5. Os demais convidados já chegaram. (The other guests have already arrived.)