After about 2 years in your fort, visitors may petition for citizenship, and accepting will allow them to have labors in your fort. They still will not be able to have occupations.
While not citizenship, sometimes a visitor may come with the intent to stay in your fort on the long term. All types of visitors can do this. When a visitor petitions for residency, they will meet up with your mayor.
When you accept them, they will be added to the list of civilians in your fort as if they were a migrant. Their needs, preferences, and thoughts will be visible as normal, but they will not be able to have labors or occupations assigned. Aside from the reason why they came to your fort (such as performing in your inn), they may take brief breaks and do stuff in other locations in your fort. This includes visiting temples, libraries and meeting halls.
In a temple, if the area is defined to any specific deity, the petitioned citizen will pray to their own deity, which is sometimes of varying cultures and not those within the dwarven sphere. In a library, petitioned citizens will read books and scrolls, but cannot make them unless they are scholars.