After a ton of trial and error I figured out the fix! You’ll have to add them as a friend on EAconnect. When you’re in the main menu of the game click R3 or RS, to pull up EAconnect. Go to player search and search the username of the player you’re trying to invite. Once they accept your friend request they should pop as on option as a friend when you go to members in your league menu and select invite from friend list.
My league was not showing up in searches and I couldn’t invite players on different platforms. This was the only fix that enabled it for me. Also, we had no issues with people finding the league prior to the draft, so I’d encourage you to have everyone join the league BEFORE the draft to avoid the issue in the future. Not sure if this is actually the catalyst of the issue, but we were 26/26 before the draft and 0/6 after the draft being findable via search.