Good advice above. In addition, you might try brainstorming a menu of activities before a playdate, with the specific friends in mind. That gives you in vitro time to discuss the pros and cons of various play strategies, and gently guide her towards ones with a higher likelihood of success. It can be good to practice some cue words or signals, too, to help remind her to implement strategies you've agreed on. Then, once the playdate commences, stay in the area to cue her.
Or you can try inserting roleplay, maybe with older sister's help, to practice the collaborative play strategies you've talked through. In between planning and live play, that is.
We have a standard for play in our house which includes two key principles: 1) Every child present must be included, even nonverbal ones (of course, they have the individual option to abstain, but the design of play has to be fully inclusive, and they must be welcomed and invited). 2) Unless everyone is happy, it's not play. Corollary: if you are happy (e.g., because you got your way), and everyone else is upset, then you are happy at their expense, which is essentially bullying/victimizing the others.
On the one hand, this is somewhat vulnerable to sabotage by a child who decides to sulk every time, de facto eliminating the state of "play" for everyone, but we have found that, with gradually faded adult coaching, they (at least the older ones; we're still working on the tail end) have learned to negotiate mutually-acceptable play for a wide range of ability and developmental levels, because the explicit expectation is that there is always a way for people to find a common enjoyable interest. It takes a lot of time, a lot of coaching, and a fair amount of quiet parental hair-tearing...
