I know that you may be frustrated with this young lady wanting to be friends with your group, but in al honesty if you have already tried to get her to understand that you do not want her coming around.
Then you may want to consider meeting with your group some place else.
Clearly you already know that she is mentally challenged. So you will not be doing the right thing to reject her harshly.
I agree with Deborah went she said “No wonder the highest cause of death amongst Asperger’s is suicide.”
If at all possible get her to come with you so you can speak to her separate from the group and then speak with her in a gentle way to see if she will understand that you and your group are meeting like this because of a group discussion working on a project and that as a group you need the privacy.
At this point you may see her reaction and you will need to ask if she understands you.
If this does not work then you and your group will need to find another place to meet.
Hope this helps. Autism success stories: http://www.glycoresource.com/autism.html
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Robert C US