I always think that direct, honest communication is the best way. Talk to her, let her know you want to be in the loop. All three of you sit down over some tasty beverages and talk and talk and talk until you're tired of talking. Let her begin to feel safe confiding in you as well. Let her know there is nothing she can say to one of you that she can't say to the other. Reinforce the idea that you and your partner share everything. Build up that trust. Find out whether you guys want a V or a triad. Talk about scheduling. Nothing should be off the table.
Really, nothing can be lost be open and honest discussion. You may even discover some aspects of this you previously overlooked.