How i can enable the field of a particular Question Based on the UserLogged In appian. please help

Please elaborate. What questions? You can check whether a user is in a group using 

docs.appian.com/.../fnc_people_a_isusermemberofgroup.html

Additionally, this will get you the account of the logged in user:

fn!loggedInUser()

a!textField(
label:"show only user is admin",
value:ri!valueTosave,
saveInto: ri!valueTosave,
/* you can add any condition here to show or hide it*/
showWhen: a!isUserMemberOfGroup(username: loggedInUser(),groups: {
"constant og group 1", "constant og group 2"
})
)