When using tēnā kōrua or tēnā koutou plus a term of address (such as hoa, kui, koro, hine etc) you put an 'e' before the term of address if it's short (no more than one long or two short vowels) and you put mā after the term of address.
Eg: Tēnā kōrua, e tama mā. Hi boys (to two boys).
Tēnā koutou, e tama mā. Hi boys. (to three or more boys).