My goodness, but this slumber party is filling up.
Ok, let's make an invite list for the two parties, based on what we've heard:
Foal Patrol Party:
Pyrite
Spark
Smoothie
(Dawn? She's clearly invited.)
I've considered inviting Garion, actually.
Others
Purple and Foxglove's Party:
Purple
Foxglove
Shelly the turtle
Foxglove the fox
Gearstride
(Silvermane? Dawn Blossom? Both would be invited by Foxglove, but I doubt Hawk wants two characters in the same scene. Not to mention schedule overlap.)
(Soft Serve? I think Foxglove would invite her, but I can't remember if she's being retired still or not)
Others?