well, simple press only knows about new members if the wp api is used for creating new members… if that plugin is not using the wp api, then normal user creation hooks are not fired and other plugins are not told of the new user… not much we can do if the plugin fails to follow the wp api…
you can check this by finding the user in the wp users table in the db… then see if there is a corresponding row in the sfmembers table…
however, when the new user visits the forum for the first time, the forum will see the imbalance and create the sfmembers entry and the user will get access as determined by permissions…