Is it necessary to register user email in manage API for front end user , from back end while working on API??

Web API in GrandNode is for administrator, not for frontend users. If you want to extend the functionality of GrandNode WebAPI, we can create such features for you. Just use the developer support services.

but api works only after register on manageapi whatever the user role ?
i want some api access for all user.

shortly, i dnt need api will work only after registration on manageapi parts?
is it posible or not in grandnode?

Out of the box it's not possible. As I said it's not an API for all users, but for administrators added in "Manage API users".

Jumping into this thread with a follow up question.... Is it possible to link a web api user to a vendor, so a vendor can manage their products via the API but not access other vendors products?

It's out of the box, but API is possible to extend by Plugin.
Regards
Krzysztof