The company that a user opens to is set up in the Set configuration programs, either UT-E or TAS-C.  At the bottom of the general Tab, Startup Company-- be sure to include the B before the company code (if your company code is BC, put BBC in the startup company).
As for access to Change company TAS- C and TAS-H, you need to have them in the list of available programs for that user in TAS-G under system manager-TAS.

I cannot figure out how to give access to TAS-H to some of our users when none of the TAS items in System Mgr show up at all in the PS-B Master System Administrator(Security Level 0).  All of the modules UT through DE show up on the screen but no TAS.
 
I went to the Maintain Menu Access (Tas-G) and made sure that I (the master system administrator - Access code CHARLENE) had access to those the TAS-G programs, just in case.
 
Any other suggestions?