Adding a domestic partner

Hello~

 

I may want to add my fiance as a domestic partner to participate in my insurance/dental plan at work.  He has two additional dependents on his insurance now, so that would be three people total added to mine for a family plan.  First, I don't think they'd be able to be tax dependents for me, would they?  And if not, the added cost per month is $1,130 for medical and dental.   I assume that would be taxed every month?  Would you be able to tell me the amount of tax that would get taken out of my check?

 

Thanks!