Join party in progress

[Seig345]

It's because there's no groups that need to be filled.

This, because the option simply means you're willing to fill in for a missing spot if one arises before you're slotted into a fresh party. It doesn't mean you'll only be put into in-progress parties. Otherwise everyone in a hurry to get their daily bonuses out of the way would sign up for that and queue times would be longer.