It is permissible to deal with them with the intention of calling them back to the true principles of Islam. There is also no problem in eating with them with the provision that one has to abstain from eating anything that is unlawful in Islam such as pork or meat not slaughtered according to the Islamic shariah. Animals slaughtered by members of this sect are forbidden for Muslims.
[Note: For other opinions on this topic search: Ahmadiyya.]