I want to know if it’s grammatically correct to say, " I would let her know that you would join her "
Can you say this after asking someone if they would like to accompany another person out and they agree to do so.
If you want to suggest that they should contact her directly, your sentence is almost correct, but you need to add two words:
I would let her know that you would like to join her
If your want to say that you will contact her, the sentence should be:
I will let her know that you would like to join her
"I will... " means that you will do something.
"I would..." is a polite yet informal way of suggesting that you think that somebody else should do something.
"would like to..." is a polite way of expressing an interest in doing something.
You could replace "would like to" with "will", but that is less polite.