@wiwarez Please start a new save and see whether the problem is still present. If it's not, switch to playing a different family within your current save and test. You don't have to save your progress; just see whether sims show up when invited, then quit without saving. Let me know what happens.
If new sims in a new save can't get their friends to show up, please move your entire Sims 4 folder out of Documents\Electronic Arts and onto your desktop. Launch the game, which will create a clean folder with no content. Don't add anything back to it; just start another new save and test.