Not sure how much to withhold in social security if the rate is 6% on the first 100,000 and they earned 105,856? If they earned less than 100,000 I can do it.

$5856 has gone over the limit and no longer gets taxed. If this is being figured for the entire year, you figure it on the $100,000 as that's the limit subject to the tax. If you're figuring it on an individual paycheck, I cannot tell you because I don't know how much the paycheck is. If it's under $5856 -- that is, if they hit that limit prior to this pay period -- then nothing is withheld because it's already over the limit. If the current pay is over $5856, you need to know what prior year-to-date was and figure out how much is left that is subject to S.S. i.e. if prior year-to-date was 99,900, then 100 is still subject to the tax.