In my experience, Yes, you need to be a member first. This usually involves opening a Savings Account with an initial deposit amount (ie.. PenFed requires a $5 initial deposit). Many Credit Unions now allow you to set-up the account online; they will mail you the required paperwork they'll need you to sign.
The benefit of Credit Unions is that they almost always offer superior Credit Limits on their credit cards.
Do you need to be a member to get their Credit Cards? I hear everyone rave about CUs. What is so great about them?
Some credit unions will allow you to be approved for credit card prior to membership (i.e. UFCU).
Benefits of CUs include:
No Balance Transfer Fee
Same interest rate on cash advance as purchases (2-3% transaction fee)
Less probability of AA
Depending on credit score, great interest rates and CLs