Dear D:

You wrote:

> Dear Sirs:
>
> Can you please tell me which is the correct pronoun usage below?
>
> “Dear Joan: it was a pleasure to meet with Dan, Dennis and yourself last
> week”
>
> OR
>
> “Dear Joan: it was a pleasure to meet with Dan, Dennis and you last
> week”
>

There is no reason to say “yourself.” “You” is sufficient. “Yourself” is used either reflexively (when you do something to yourself as “I gave myself a bath”) or intensively (when you want to emphasize the individual
accomplishment or status as “He did it all by himself”). Also, any pronun with “self” needs an antecedent. “You” does not have one here.

For more see https://englishplus.com/grammar/00000027.htm “Pronouns Ending in -Self.”