How to tell this girl we don't want her to come?

So tomorrow me, my best friend (rosie), emily, brittany and JESS are going up for this walk/picnic, but Jess just follows us all the time, and she get's on our nerves, we didn't even invite us she just said "So I think I'll be aloud to come with you tomorrow." She ALWAYS says things like this, but we don't want her to come, but we don't want to tell her and hurt her feelings, she knows where we're going, where we're meeting and the time!