If someone invites you to a group collection, you can join it via the iOS app. Please note that as you have to sign in to the app use this feature your contributions will not be anonymous.
To join a group collection
- Open the app
- Select the Groups tab from the home area (see screenshot below)
- Click Join Collection
- Choose whether to join by scanning a QR code or entering a text code
And that's it! You can find it under the groups tab, and you can add items to the collection whenever you want.