After creation, you are automatically the faction leader. To add more nations, open diplomacy with any eligible country and select (costs additional PP, usually 20-50). Success depends on:
If a nation is being threatened by the same country you are fighting, they will likely seek the safety of your alliance. hoi4 how to create faction
All members automatically share docking rights and military access. After creation, you are automatically the faction leader
Scroll down the list of diplomatic actions to find Green Check: They will join immediately. hoi4 how to create faction