Scenario:
- FC member was at the final boss of Sohm Al (Hard).
- Their tank was vote dismissed for whatever reason
- Asks in fc chat if anyone would be able to select Join in-progress Sohm Al to fill the spot (because at 3.30AM waiting for a tank who had JIP enabled could take 20 mins or so)
- I do just that, but it seems the duty finder does not prioritise in-progress duties as I was put with a brand new party at the start as normal.
- FC member died in his chair while waiting for a tank. RIP not completely true

First things first: enable the FC window to be opened while in duties (same goes for friend list I think is also disabled)
Then: Allow right click "Invite to party", if person is not eligible (e.g wrong class/low ilvl) grey the option out
That player gets message "Join <name>'s duty in progress? Yes/no"

... and that should solve the problem of parties with open slots waiting a disproportionate amount time to finish what would take about 5 minutes.