How do insurance companies send short messages to invite customers to tea parties?

Don't send such messages, it's not formal. The content of your invitation letter is roughly shortened to introduce the company's situation and address, and the middle part thanks the customer's support. Finally, you or Mr. and Ms. XXX are invited to attend an academic conference or discussion at XXX address at XXX time. Finally, it is XXX time.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.