"Hello Iain and Anna, come in."
Translation:Halò Iain agus Anna, thigibh a-steach.
It depends on how many people you are talking to. You always need to use the plural imperative ending if you are talking to more than one person, regardless of how formal/familiar/respectful you are being. If you're talking to only one person and you would usually use thu with them, you don't use the ending, if you're talking to only one person and you would usually use sibh with them, you have to use the plural ending.