To ensure voluntary participation, you should inform your respondents that participation in the survey is completely voluntary and that they may withdraw from the survey at any time. Your invitation to participate and the introduction to the survey are good places to include this statement.
At times, this simple statement may not be sufficient to ensure that participation is truly voluntary. Some respondents may fear that they will be negatively impacted if they do not participate. For instance, if an instructor asks students to participate in a survey, some students may fear that nonparticipation will impact their grade.
To avoid this, assure respondents that there is no penalty for refusal or withdrawal.