I was wondering, if the main account holder can read or get any info from another phone on the shared plan?
The account owner can see the call logs (numbers dialed and when), text message log (numbers and when the messages are sent or received) and the data amount including when the system records the usage. The account owner doesn't have access to the contents of the text messages.
Is there anyway to block that from happening?
No. You could set up your own account or get a prepaid phone if you'd rather not be part of a shared account.
Since the account owner is (financially) liable for all the activity on the account, the account owner should be able to see what's happening. Having your activity viewable by the account owner is a tradeoff for a lower bill because you are sharing the account.
As tikibar1 said, the way to prevent that info from being viewable is to set up your own account.
Does that mean they can read my emails as well? I'm just comparing what
would be better staying on someone's shares account or just getting my own
