You will want someone who has their finger on the HR needs starting day one; someone who understands the paperwork needed/insurances needed or at least someone who understands when they don't know what to do and can call in reinforcements!  That person can outsource the need or handle it themselves if they have experience/knowledge with the specific need or task.   

As stated by others in this group, there are some great outsourcing solutions.  Many HR Consulting firms have a dedicated HR package on a monthly retainer; others bill on a project rate or will bill hourly for the work done for you, then be available on a reactive basis (you call them when needed).  When you are a start up or a small company, spending even $5000 a year with an outsourced consultant is better than $90,000 for 1 staff member, and even better than paying out thousands in fines because "you didn't know" or $5,000,000 for a lawsuit that could have been easily prevented by someone who knows how to protect the company... by protecting the staff!  

*Pointers on choosing a firm!  (1) Choose a consultant in YOUR STATE so they are in touch with your needs.  Choose a consultant who has experience with start  ups and small business - not just large organizational strategists.  Someone who can roll up their sleeves and DO the work rather than those who only tell you what you need to do.  (2) Make sure they have a decent sized active client roster; someone who works with just 2 other clients may not be the right choice.  Make sure they aren't just "unemployed" at the time, and will stick around when you need them later on.  (3) Ideally a consulting firm with a team will be a better choice so you don't have to wait too long for help if they are on a project or with a client and can't take your call or respond timely when you reach out.  (I've been the solo person when I first started my firm, and it's rough when being pulled in 5 directions at once... then the following week have nothing to do!)  And (4) ensure they are properly insured, have a contract, have a website.  They don't necessarily have to have a brick and mortar office, because that's high overhead and higher prices, but they should be established.   You tell the Firm what you want from your relationship, and try not to be forced into a box or given only 1-2 solutions that don't work for you.  You should be able to create the relationship you want with them that fits the reasonable budget you have.

Typically an organization will have a dedicated on-staff HR employee (part or full time) when the organization first reaches between 50-100 staff members.  Consider a ratio of 1:75 when adding to HR staff.  Payroll should be done or closely supervised/approved by HR.  For checks and balances, a finance person should oversee payroll when done solely by the HR function.  Many larger companies have HR generalists on staff, and will still outsource projects to specialists and strategists.  A good HR firm will grow with you and adjust their relationship with you as you bring in-house staff to do more.

Good luck!