Enable party invite popup
Find your Neverwinter Nights directory and locate the file
You can open this file in any text editor to edit it (for example in Notepad).
The file looks something like this (with minor variations based on your client settings):
<poem>[Game Options] Single Player ItemLevelRestrictions=0 Single Player Enforce Legal Characters=0 Memory Level=1 Memory Access=2 TooltipDelay uSec=100000 Enable FloatyTextFeedback=1 Hide SecondStoryTiles=1 MouseoverFeedback=1 TextBubbleMode=1 TargetingFeedbackMode=2 Difficulty Level=1 DisablePartyInvitePopup=1 [Server Options] (lots more stuff below that is not really relevant at the moment)</poem>
Somewhere in there, between the Game Options and the Server Options, you need to find and change this line:
Save the file and close it. You are all done here (assume it's done correctly). You will now have to restart your NWN client for the change to take effect.