Communist China refused, although the game was historical. Historically they accepted. And no decisions appeared to invite the others, I looked for it.
In that case, I believe you're actually screwed. The warlords can't ask to join unless Communist China is in Chinese United Front.
But again, these are decisions the WARLORDS take. You do not invite them; they ask to join.
To get more in depth on what went wrong: (which is frankly not behaving as designed) If you take the focus to form the United Front, you create a faction and give Communist China a regular invite, which they either accept or reject based on how much they like you. Historical/Ahistorical won't guarantee anything here.
However, there is another way to form the Chinese United Front. (this is the way the AI does it on historical) If you avoid taking the focuses "Foreign Threats" and "Prioritize the Interior," a warlord will eventually ask you to meet with them. If you go, they will ask you to enter talks with Communist China to form a United Front. If you disagree, the warlord, on historical, will kidnap you and force you to enter the talks. (If you go willingly to the talks, there's a 10% chance Communist China will reject the United Front) After, Communist China will *always* ask you to form the United Front if you are on historical. (they might just execute you on historical) *If* you accept their offer, you will form the United Front and Communist China will automatically join.
EDIT: The step-by-step guide to 100% making sure the United Front forms on Historical as China: (this is what the AI does)
1. Don't take the focuses "Foreign Threats" or "Prioritize the Interior" until after the United Front is formed
2. Wait for a warlord to ask you to meet with them
3. Agree to meet
4. They will ask you to ally with the Communists
5. Refuse to do so
6. You will be arrested by the warlord and forced to meet with the Communists
7. Don't declare war on the warlord for this
8. The communists will offer you to form the United Front
9. Accept their offer; this will put you in the "Chinese United Front" faction with Communist China, with you as the faction leader
(now you can take "Foreign Threats" or "Prioritize the Interior")
10. Wait for each of the warlords to ask to join the United Front
11. Accept their requests (if you reject, they will keep asking, though)