Simple enough function, couldn't see it suggested elsewhere.
So you use Party system a lot with premades, or let's say you host tournaments or gamenights or whatever.
Annoying to re-enter all those names, right? Don't have every user possible befriended because they come and go and it wouldn't be logical to add hundreds of users who you might see only once or twice in a plethora of users?
Open Party system page, there would be a third box in the invite sub-menu with a cached list of the last say 15 online users who accepted a party invite from you, friend or not. On this list by each username is the simple invite button as usual that moves them over to the far-right Invite column with the big INVITE button beneath.
Spoiler:
Why the suggestion? Why not just add people?
I'm in a ToS-centered discord server with 4k members with a moderate amount of gamenights, and as you can imagine with 4k people, you might not see the same person twice and your gamenight may go on for hours and have to re-invite people each and every time.
Over time, your friends list would be unimaginably big and tough to sift through, and re-entering usernames each time can be a pain for some.
I can only imagine what tournament hosts go through with planning.
So having a quick alternative to re-invite the last several people who have accepted an invite from you saves considerable time and tedium.