2 bugs with multisite !! please help

Thanks for your reply Leslie.
What I mean is I am testing my website now. I act as a user not admin or super-admin.
Here the case for example: if user registers in subsite A, he/she can add listing and make payment in subsite A, but he/she cannot add listing and make payment in subsite B. and it happens vice versa.
From what I learnt from Luciana’s link above that user can only add listing in subsite A without any error if he/she is registered in subsite A. and if he/she wants to add listing in subsite B, he/she has to re-register again in subsite B.
What I want to ask is how to tell the user that he/she needs to register again in another subsite. Maybe Luciana has resolved this problem in her website.