How would I add a line in the client letter and joint letter.

That's really straightforward and exactly what I've done with my own client letters. When you're in the actual letter on your computer, right click and choose "Edit template". Then go down to the bottom of template and add what you want to say. I believe the original template already reads "If you have any questions about your tax returns, please contact me at {T1PreparerPhone}. You could add your message somewhere in that line or add a new line altogether. That's up to you.

When you click the little "x" in the upper right corner to get out of the template, a pop up screen will say "This document has been modified. Do you wish to save your changes?" I think it then asks you if you want to use this as a template for all future returns, or something like that. My memory is a little fuzzy there.

To add to Jparson's answer, if you want the software to include a specific year to that statement, you will have to add a year somewhere in the tax return as well; I'm assuming not all your clients started using your services at the same time.
Whereas you could use the Personalize option, that's very time consuming and defeats the purpose of automation. Without knowing your return, I'm going to make a suggestion and we can always change the particular field to something else if it's already in use.
Under Office Use on the Information form, you have a free format areas. Assuming you have one of them not in use, you could use, for example, Client reference # to enter the year they joined you. From here you would do what Jparson's instructed, and the line you would enter would look something like:

Thank you for being our client since {T1Info.S[38]}

If you would rather use a different field, you can right click in it, choose Copy field code from the context menu and pate that code instead to the end of your sentence.

Just to add one more point. If you don't want to add this line to all your letters, you can create a conditional statement:

{If T1Info.s[38]<>""}Thank you for being our client since {T1Info.S[38]}{endif}

Again, the coding here T1Info.s[38] will change depending on which field you use.

Thank you very much Mr. Parsons and Fab. You guys are the best. I really appreciate it.