Find another play group or Mommy group, that IS your daughter's age.
It has nothing to do with how "smart" a child is, a child is a child. AND "emotional maturity" and "logic" based emotions/rationale is still not fully developed/understood in a child this age, no matter how "smart" or verbal.
Older kids, will OFTEN make the younger kid do things like that... they get relegated to being the "gopher" or made fun of. It will NOT "make" your child a 'victim'... and as a Mom, you just teach and explain social ideas to them. So that they know the difference between social innuendos.
But, I would definitely find a playgroup that is your daughter's age. And yes, it all has to do with age-set development. MANY smart kids, are STILL their age.Jumping ahead as an example... my friend's daughter skipped a grade because she is "smart" (academically) BUT EMOTIONALLY and per maturity, she is not on the same level as the older kids... and she is often left out and is not going through the "same" development or phases as the older kids. THUS, she is often feeling "alone" or is teased.
There is nothing 'wrong' with your child. That playgroup is just NOT A GOOD "FIT."
With my daughter, (who is very smart, emotionally smart, and verbally articulate), we from the time she was a toddler, showed and explained MANY social situations to her. As she has grown up... she has become a VERY wise little girl... and is more emotionally "mature" than her same age peers. BUT, she is still just a kid. AND, she has fun no matter what as a kid her age with friends her same age. BUT, she can navigate herself through social situations or yucky situations, very well. AND it does not affect her confidence. So.... it is also how a child is taught/coached about many situations, and innuendos... and that she can be her own person.
ALSO, not all "older kids" will treat younger kids this way. Some older kids, will stand up for the younger ones, and include them too and be 'kind.' BUT, as a "group" mentality... older kids may leave out younger one's or make them the "joke." When my daughter is playing with her friends, and my younger son is with them.... she WILL stand up for him if they make fun of him or tease him... she will not allow the older kids to treat him like a 2nd class citizen.
Sure, kids will be kids. AND, that is how kids learn about life... that not all "friends" are nice or kind, and they won't always be fair, and they can't just "get up and get a new group" when there is a problem, etc., etc. But at the same time, since you can, I would just find another play-group or Mommy group that is more pleasant. There is no "rule" that you HAVE TO just stick to one playgroup no matter how unpleasant. (AND, it will teach your child all about "problem solving" and how to take action and FIND what DOES 'fit' well, which is also important for kids to learn... not just "putting up" with life. BUT Rather, that YOU can "create" what is more appropriate.)
Just because your daughter's language abilities are so high, as you mentioned, does not mean that she is always going to have to be with older kids. Think about her AGE-set... and OVERALL development. There are kids at my daughter's school, that are in the gifted classes... but hey, they STILL are their age, and STILL hang out with friends their own age.
Try doing a Google search about play groups or Mommy groups in your Area. Then that way perhaps, you can find one with the appropriate ages.
All the best,
Susan