From LimeSurvey Manual
By default, an user (non-admin) cannot grant survey permissions to users that are not part of the same group as the survey administrator. This is a security option enabled by default in LimeSurvey. To change this, you need to deactivate option Group member can only see own group, located in the Global settings, under the Security tab. However, if you feel unsure about disabling this option, you can create groups containing those users that can be seen and be granted survey permissions by a survey creator.