General Brazilian
unir-se com; unir-se a
most common usage in everyday Brazilian Portuguese
Rio de Janeiro
unir-se com
colloquial use, sometimes shortened to 'se unir'
São Paulo
unir-se a; aliar-se a
business and formal contexts prefer 'aliar-se a'
Portugal
unir-se com; aliar-se com
formal contexts often use 'aliar-se'