I invited 3 players to spring training as invitees without contract. Based on performance I decided to keep them and added each of them to the 40 man roster, which caused their $0 salary to become $55000 and they now show up on the 40 man.

however now I cannot assign/promote them to any team, I am not sure why?..

Any thoughts/tips? I want 2 assigned to the ML roster and the third with options assigned to aaa.

Don't know if there is a solution from where you are now. Next time, offer those players a minor contract with STI instead of just an STI... they will accept and then you can assign them.

I believe you can just release the player and he goes into the free-agent pool. You can sign him there to a regular contract and assign him then. But I do what dedelman suggests and sign the player(s) to a minor-league contract.