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

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.