I work with children with speech and language delays, and I've seen this. One theory is that she doesn't know how to enter into play in a positive way. Or it is possible she doesn't know how to play. What you can do as a parent (with the preschool and Y teachers) is try to figure out what is "motivating" the behavior. Then work with her, in the situation, as well as role playing to help her learn the right thing to say and do. So if you think she doesn't know how to approach a peer and ask to play, you or a preschool teacher, go with her to the friend. Tell your daughter what to say, and have her repeat it to the friend.
Since you are considering evaluation, I would say that the "problem" is severe enough for you to go and do it. Depending on your state, free evaluation can be completed by either the school district or Health and Welfare. In our school district we have screening one day a month where 3-5 years olds (by appointment) go through our screening or basic evaluation process. We look at skills in physical development, thinking, social interaction, self help skills, speech and language, as well as hearing and vision. If a child scores lower than desired in one or more of the areas, we send that child's file on to a school who does a more thorough evaluation to determine if they qualify for services. Services may simply be speech therapy, or it may be more comprehensive as a part of a preschool program in the public school (other districts partner with Head Start). If you need help finding the screening for your daughter, ask your doctor, call a Head Start program, call the school district, or call Health and Welfare and ask about an "early intervention screening".
I would try the above described evaluation first (it's free). If your child does not qualify for services (a child must be really low) then consider medical therapy that does not require a huge delay for services.
It sounds like your child could have a "social" delay of some kind (very common with speech delays) and enrollment in a program that develops and individual education plan to increase her ability to interact appropriately with others would really benefit her. By getting her the help now, it will be easier to teacher her and she will be less likely to need the help in the future.
I hope this all is clear, sorry it is so long. I wish you the best, and please post back if you have any questions about what I wrote.
State Certified Early Childhood Special Education Teacher
New Mom as of March 2009!