First and foremost -- your younger daughter needs to be otherwise occupied. That will help a lot for the short term. Schedule a play date for her at another family's house and then have a playdate for your older child at the same time at your own house. It can also work to have two playdates at once at your own house-- younger girl has a friend over, older one has a friend over -- but frankly I think that's going to end up even worse, with your daughter's friend playing with BOTH the other girls while your daughter cries on her own. I'd really make an effort to get younger sister out of the house, or to have a play date while younger sister is at another activity. If your husband or significant other can help by just taking younger sis out and away during big sister's play dates, that would be good too.
Both your girls are just a bit too young, to me, to navigate this kind of thing emotionally yet with anything more than upsets; your older child needs to build up her playdate skills first without the added stress of her guests having an instant, different play mate available. Nobody's at fault here -- it's just the way things are, and in a few years the older girls are not going to end up with the friend off choosing to play with younger sister, but at their current ages, there isn't enough difference between the older guest and your younger child for that to kick in yet.
Your daughter does not intentionally mean to be bossy or mean to her friends, so please take care that in your own mind you're not labeling her that way or over-thinking it as "Oh, no, she's going to be bossy forever and never have a friend." Step back for a moment and think about how she's just now getting started on the K playdate thing and doesn't quite get that not all kids whom she likes also want to play what she wants to play. That's a normal thing and does not mean she is going to turn into a bossy queen bee; she's just being five and at five, kids want to control things around them.
It could help to role-play with her before a play date: "What do you think you will suggest that you and Sally play?" "What if Sally says she would not like to play X but she wants to go outside to do Y instead?" Have her anticipate it a bit. She will likely forget, in the heat of the moment, that you talked about it but you can try, "Remember how we talked about what to say when our guest says I want to do Y?"
Then have a plan in place with your daughter: The girls will do Y for a while then will move to X. Use a timer for sure! That puts the timer in charge, not you, and often, young kids will respond better to a mechanical device telling them time is up than they do to an adult, with whom they can plead or argue.
You may need to take more of an active role in playdates than you may think, temporarily; if you tend to tell the girls, "Go play," that can be overwhelming for a young kid and can lead to the tensions you describe; try asking them both at the start what they plan to do; let them hash it out in front of you but only very briefly; then say, "How about X minutes of Sally's idea and then Y minutes of Daughter's idea" and use the timer. If they agree right away there is no need for all this, of course, but keep an ear out for problems.
Always, always have your own alternative plan and activity ready to go. It can break the tension if you are able to announce cheerily when you see trouble brewing, "Hey, girls, I have cookie mix made up here -- do you two want to put it on the cookie sheets for me and in 10 minutes we'll have cookies!" Or have a small craft ready to go --the supplies right there at hand so you don't spend time digging them out: "Girls, I have these pretty beads right here -- if you would like to take a break to make bracelets, do you want to do it now or later?" Your daughter may still fuss if she sees you as invading her play date; only you know whether these interruptions to break tension will work with her or not, but I found they worked well to save play dates that were getting tense or dull.