You'll have to use a sf::Clock and create a delay before adding the next character to the sf::Text to get a scrolling effect.
Yea but there is no sf::Text::push_back, so I suggest he needs to create an std::string (yes, it's
"an" before an abbrevation due to the pronunciation of "
es", the first letter in std) where you push back each character.
After that, you use sf::Text::setString(std::string) and redraw the text.
Of course you use a delay like exploit3r said, or you could make it dependent on screen refresh rate. sf::RenderWindow::setFramerateLimit(unsigned in);