Can we use email or string as a primary key in database table?

Yes, you can use string as primary key but it will slow down indexing so it is always better to use int as primary key.

If you have many users in database table I recommend you to use int as primary but if it is necessary you can use string and yes you can pass it to MVC ActionMethod.

please note: string must be unique, it cannot be email id , but it can be unique guid, like we see in default asp.net User table of mvc 5