Limiting Input Box Selections

I currently have an input box that asks for a user to enter their intials. Is there a way to make sure that the initials they entered are of a valid user. I only have about 6 people that use this. I would like it to check their input against the list and then if its not right make them enter them again.

Sender = InputBox("Please Enter Your initials:")

Example: CJH, PHT, FXH, TOM

If the entry is not one of those then I would like it to ask them to input again.

Once the input is made it outputs to a log sheet for me.

Windows("RCSN Log.xls").Activate
Sheets("Log").Select
ActiveSheet.Cells(ActiveSheet.Cells(Rows.Count, "A").End(xlUp).Row + 1, "A").Value = Sender
ActiveSheet.Cells(ActiveSheet.Cells(Rows.Count, "B").End(xlUp).Row + 1, "B").Value = SendDate
ActiveSheet.Cells(ActiveSheet.Cells(Rows.Count, "C").End(xlUp).Row + 1, "C").Value = SendTime

I would then like a yes/no input box that asks them a question. If they enter yes I would like it to output to the log. If they enter no I want it to just continue on with the rest of the macro.