That would be lovely, thank you.

Responding politely to invitations (whether accepting or turning them down) is important in work situations.

To accept an invitation, say yes, and thank the person:

Would you like to come to lunch with us?
Yes, please. That would be lovely, thank you.
Yes, I would. Thank you for asking.

If you can’t accept the invitation, it’s polite to apologise and give a reason:

Would you like to come to lunch with us?
I’m sorry, but I can’t. I’ve got a meeting in half an hour.
Thank you for asking, but I can’t. I’m already meeting another colleague for lunch.
I’m afraid I can’t. I have to run a couple of errands. Perhaps another time?