和写中文的邀请函差不多，只是直入主题不要有那么多寒暄就好。说清会议的名称、地点、时间安排等然后让对方确认出席。一般邀请应在会议开始前1-2周发出，最少也要给2-3天时间让对方给出回复。因为你没有给出具体内容我简单写个格式：
(收件人信息） Receiver's detail
Dear Mr. XXX,

We'd like to invite you to attend the conference regarding xxx. The conference is held by xxx, and the address is as below:
Adress detail
The conference will be start from YYYYMMDD and finish at YYYYMMDD, start from XXam everyday and finish at XXpm.

Please confirm your attendence with us at your earliest convenince. In case you have any further query regarding the conference detail please feel free to contact us.

Kind Regards,
XXXX
Sender's detail（邀请人信息）