The basic ruling is that da`wah includes all people including Muslims and non-Muslims and of course persons of the opposite sex. However, when observing da`wah with a person of the opposite sex, both parties should abide by the Islamic ethics and guidelines of relationship between both sexes.
Dr. Salah Sultan, president of the American Center for Islamic Research, Columbus, Ohio, and member of the European Council for Fatwa and Research, states: Obviously Prophet Muhammad (peace and blessings be upon him) practiced da`wah to all, humans, and Jinn. There is no doubt that the Prophet (peace and blessings be upon him) gave da`wah to women.
As for giving da`wah to members of the opposite sex, the natural and ideal way is to start da`wah with people from the same sex, and then you expand your circle. However, when the case involves men giving da`wah to women, they must follow the following guidelines:
1. No khalwah (Arabic for: seclusion) is allowed.
2. Talks and conversations have to be serious and definite.
3. Women should not speak so softly that they can attract the opposite sex.
4. Da`wah has to be done sincerely for Allah’s sake, and not for any worldly cause such as looking for a wife/husband.