My button on Tkinter doesn't show up

Question

I've recently decided that I wanted to have a play about with Tkinter, however I am trying to have an 'Enter' button to send me to the next window once a piece of data has been put into the entry but my button doesn't show up and the program tries running the command I have put into the button statement. Whenever I remove the command the button shows up but clearly does nothing.

Please note that I am fairly new to Tkinter so I may be doing some things inefficiently.

if os.path.isfile(NameDict):
    Login()
else: 
    rootS = Tk()
    rootS.title('Authentication')
    rootS.geometry('400x300')

    Name = Label(rootS, text = 'Please Enter Your Full Name\n')
    Name.grid(row = 0, column = 0, sticky = N)

    NameIn = Entry(rootS)
    NameIn.grid(row = 0, column = 1, sticky = N) 


    #this is where i am having the issue       
    NameB = Button(rootS, text = 'Enter', command = CloseName(NameIn,rootS)
    NameB.grid(columnspan = 2, sticky = W)

    rootS.mainloop()

The CloseName function.

def CloseName(NameIn,rootS):
    name = NameIn.get()

    if os.path.isfile('C:\\Users\\Bradley\\Desktop\\Log In system\\Members\\' + name + '.txt'):
        login(name)
    else:
        with open('C:\\Users\\Bradley\\Desktop\\Log In system\\Members\\' + name + '.txt') as f:
            data = f.readlines  

        Created = Label(rootS, text = 'User files has been created')
        Created.grid(row = 1, column = 1, sticky = CENTER)

        rootS.destroy()
        signup()