Prevent people from adding you to WhatsApp group
Open WhatsApp, and click on the dots at the top-right corner of the screen.
Select Settings from the menu that pops up.
On the Settings page, click on Accounts.
In the Accounts area, click on Privacy.
When you get to the Privacy menu, scroll down and select Groups there.
On this page, you could select who could add you to the groups; either everyone, or your WhatsApp contacts, or nobody.
By preventing admins from adding you to any group using the option under privacy, they can still send you invitations to join but privately.
When the invitation comes, you can see and review some of the members of the group, their names and you can see the group icon (if any) and about the group. Sure you can always leave the group too when you want to.