Dear …
We are pleased that you have made the decision to join our __(company/firm/team/etc)__. It looks like it is a good choice for both you and __(name of organization)__. Your __(experience/expertise)__ will definitely add value to __(this project/our team/our firm/etc)__ and this __(type position)__ position is certain to offer you the challenge you are looking for. We are all looking forward to working with you.
As you know, __(I/Name)__ will be your direct __(manager/supervisor/etc.)__ and will be able help you with most of the questions you have about this department and your position. You will also find that your __(colleagues/co-workers/etc)__ will be very helpful resources. For those questions you may have regarding __(company policies/benefits/payroll issues/etc.)__, we encourage you to talk to our Human Resources department.
On behalf of __(company name)__, I want to take this opportunity to welcome you as the newest member of our team and wish you every success in your career with us.
Sincerely,