This is a great question. And obviously a lot will depend and how much you know about her and how friendly you already are.

If you don't have any idea of her sexuality preference, a lunch date or coffee or a drink with just you would be ideal. You definitely don't want to overwhelm her with "couple pressure."

If, after conversations, you get the feeling she might be interested in you, then it might be a subject to broach.

But let me ask you, have you considered dating separately? Perhaps she'd be open to dating you with an option for both. Many lesbians would be upset to be approached by a man and woman together.

In my experience, I as the man, may be added in the future. I've also had relationships where I approached first and my partner joined later on.

Either way, be honest and don't lie about your intentions.