Click Share › Invite people. Face is chosen in the oval at the top. Pick the event from the dropdown.
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The faces tagged in that event appear right away. Click the one you want to invite.
Tip: Faces with a grey ring (Guest, Invite sent, Invite opened) can't be picked: they already have access or an open invitation. A gold ring means they have an account, so the events are added to it.
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Enter Their name (filled in for you when the face has a name), Their email, and an optional personal message.
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Optionally tap a pool or tick extra events. The event they were tagged in is always included.