Here are sample responses:
Accepting:
Dear Mr. Robinson,
Thank you for the kind invitation to Games Power Company's 10th anniversary dinner party. I would be delighted to attend. Please accept my congratulations on this milestone anniversary. I look forward to celebrating with you all on June 25th.
Sincerely,
[Your Name]
Declining:
Dear Mr. Robinson,
Thank you for the invitation to Games Power Company's anniversary celebration. Unfortunately, I have a prior commitment that evening and will be unable to attend. Please convey my congratulations to the company. I hope there will be another opportunity to celebrate with you all in the future.
Regards,
[Your Name]