You have to collect 2400 event currency to send Fischl an invitation to join your party, which is just another way of saying getting her on your account. From today's domain event we get 600 event currency, same way as more and more phases of the event unlock we'll able to get the rest of the currency to get the Fischl copy.

So basically put it'll take a couple of days of playing the event.

As for the outfit, you have to collect 16 conches on the islands. 15 out of the total possible 20 are already available as of today. And as is with the currency, we'll get the remaining conches in the next few days.

For more detailed info you can open the events tab in the game and look under the 'Summertime Odyssey' and 'Resonating Visions' events. The former for the free fischl (she's available in the 'An Imperial Invitation' option) and the latter for the outfit.