Joan smiles at Ted's happiness. She loves being with him, supporting him, sharing things with him. She knows how important it can be to have your partner in your corner. His support for her looks a bit different, of course, but it's there, particularly in his support of (and delight in, really) her partnership with Sherlock.

She turns that smile to Rebecca as she invites them to the office. It's very nice of her, and makes Joan feel even more accepted into the inner circle. She turns back to Ted at his question.

"I think I'll go with Rebecca," she answers. It's not so much that she wouldn't like to say hello to the team, but rather that this outing has been wonderful but also pretty draining. Sitting and talking sounds just about right. Not to mention the potential awkwardness of walking into a men's locker room after a game.