Be upfront and ask them to join, but most of all try to consider them in all instances (like Zegai at the beginning always support him). Some have certain triggers (buy from their shop, have X character with, talk to them on several different occasions, etc) but you can not really mess up a character by just speaking to them. Generally, just ask these optional characters and they’ll join.
More or less, there are two types of recruits those who are storyline-based and that require no extra effort to get, and those that are completely optional. The order will be relative to when I got the character (or when I thought that character was accessible). Suikoden V offers a very similar recruiting system as the past four. Part of any Suikoden is amassing 108 characters, as this is a constant in all Suikoden’s. Recruit all 108 Stars of Destiny in Suikoden Five
