I used to have non family members on a family plan. Theres no way for them to check. I went in and got the phone and added it to my account. No issue.
The only potential downside is if you're on the shared data plan. Do you trust others, not family members, to stay within their fair share of the shared data pool?
Or what if they lose their job during these tough economic times, and leave you stuck with the bill? Hmmm.... Maybe it's just me being old and cynical, but I have a tough enough time trusting family members, never mind the guy down the street....