She didn’t have to tell you to add her. She didn’t have to invite you to her house party either. People don’t do that with someone they feel nothing toward. At the very least, she’s open and curious about you. That’s a good start.
Don’t overthink the first message. Keep it light and normal. Something like saying hey, nice meeting you the other night, glad you invited me to the party. That’s it. You’re just reopening the door, not giving a speech.
Avoid compliments right away and don’t jump into flirting hard. Let it feel easy, like you’re continuing a conversation, not auditioning.
If she replies and keeps it going, you’re good. If she doesn’t, you didn’t do anything wrong — you just tested the water.
Confidence here is calm, not pushy.