Add members in 24-man raid

[Legion88]

Don't worry. All party leaders of all three parties in a 24-man-raid get this message. The first who clicks decides what to do.
This pop-up asks just if you want the DF to search for a member of an open slot anywhere in your alliance it's not just in your party. it will pop up every time any player leaves/get kicked from the duty and a slot opens anywhere in the whole alliance.

For your screenshot as example it pops up because a member left Alliance C, system recognized there is a open slot and asks all party leaders "should i search for a player/s who can take this slot/s or shouldn't I?".
If you say "yes" someone you wants to join a duty in progress will get a slot and join your duty. if you say "cancel" you will never see any replenishment to your parties.

I can see why this can be annoying in scenarios where people join and instant leave.
They could just make it that DF will always search for replenishments without asking for it...