Every language has its quirks. "They" works in this case, at least in english. You can say "After they sign up". You can also say "After the user signs up", just beware the repetition.

--

In portuguese, "the user" has a gender as well ("o usuário" i.e. "(the male) user"). We usually ignore linguistic gender in most places, which causes all sorts of confusion to people coming from countries where nouns are "un-gendered".