What do you mean by instantiate? If its got an id it has already been instantiated.
An id is a property on the object, not the class. Ids are not static, but belong to instances of the class once you've instantiated them. In the constructor you would set the id, which can be used later on to get an instance of the object:
Ext.getCmp('id');
will give you a reference to the ExtJS component if that is what you're after.
If you already know that its an Ext.grid.GridPanel you can instantiate a new one:
var grid1 = new Ext.grid.GridPanel({
id: 'grid1'
});