According to Amazon, you can only share with one other adult and you must share credit/debit card info and authorizations across both accounts - so I'd only share with someone you trust - completely! I don't see anything about what's going to happen to people who already have more than one person sharing the account. Some links to the relevant info at amazon.com and highlights below:
http://www.amazon.com/gp/help/customer/display.html?nodeId=200444180
"Share Your Amazon Prime Benefits
Free trial or paid Amazon Prime members can share certain Amazon Prime benefits with one other adult by forming an Amazon Household."
http://www.amazon.com/gp/help/customer/display.html?nodeId=201806360
"About Amazon Households
Eligible Prime benefits can only be shared between the two adults in a household. Customers who are Amazon Student Prime members (free trial or paid) or invited guests of other Prime members can't share their benefits. However, other types of Prime members can share eligible benefits with those types of members.
The following Prime benefits can be shared with other household members:
Prime shipping benefits
Prime Instant Video (streaming only)
Kindle Owners' Lending Library
Prime Early Access"
Note:
In order to share content, Prime benefits, and Amazon Mom benefits, both adult account holders need to authorize each other to use credit and debit cards associated with their Amazon accounts for purchases on Amazon. This will not affect either of their current payment settings, but each adult will be able to copy the credit and debit cards of the other account to his or her Amazon account and use them for purchases with Amazon.