"please join me in thanking" is correct and usable in written English. It can be used to request the help of someone else in expressing gratitude or appreciation for something. For example: "Please join me in thanking the volunteers for their generous support.".
Please join me in thanking him for his leadership.
For now, please join me in thanking the 561,895 people that came out last September to help make the 2014 International Coastal Cleanup a great success.
We'll have a proper send-off for Gabriel closer to his last day, but, in the meantime, please join me in thanking him for his many contributions to The Wire.
Please join me in thanking Michel Gondry.
Please join me in congratulating Guru and thanking Tim.
Please join me in welcoming Andy back to our company and in thanking Pat.
Please join me in breathing a collective sigh of relief with the rest of the internet as I say, "Thank you, SkyMall!
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union