Sister wives join families either through the courting of the husband or by expressing to the family that they are interested in joining. Some polygamous groups also believe in arranged marriages. It all just depends on the custom of the family or group.
To become a sister wife, you must marry a man that is already married to someone else. You would need to find and contact a family or religious organization that believes in and performs polygamous marriages (also called “plural marriages”). This can be difficult since polygamy is illegal in the US, Canada, and Europe, but with some good online searching, they can be found. Typically in western countries polygamy is practiced for religious reasons (in Africa and Asia it is usually just a custom) so you would probably have to convert to the religion before being considered eligible for entering a polygamous relationship and becoming a sister wife.