The Masters of Behavioral and Social Sciences Intervention Program has rolling admissions from January 1 - May 31 each year.
In order for your application to be considered, you must submit a personal statement and CV, GRE scores (unless applying for the fifth year program), transcripts, and three letters of recommendation.
The Admissions Committee meets several times during that period as applications come in, and recommends deserving applicants to the Graduate School, which gives the final "ok" to accept. Applicants are automatically considered for a merit-based scholarship that is equivalent to a 25% tuition discount, and are considered in a variety of ways.
To apply, please visit the Brown University Graduate School page.