If someone who you want to add your company account already created their own company trial account, you can not invite them automatically to your account. The reason is that each e-mail can be only under one company account at any time.
We can move the user manually from one account though. To do that, contact Weekdone support at firstname.lastname@example.org stating who you want to move and under which account. You can also give this request via the live chat feature in our site lower right corner.