Discipulus Probus dixit:
Can someone explain this?
Your teacher probably originally had
salvē, amīce 'hail, o friend!' but then realized (s)he was addressing multiple people. (S)he then pluralized the noun, but forgot to make the verb agree (as others have mentioned, it should be
salvēte amīcī).
You may also use the adjective
salvus + the copula or 'being verb' in the subjunctive mode, thus:
Salvus sīs, sodālis. 'May you be well, o comrade.' (make sure that the adj. is fem. when addressing a female,
salva sīs). You could also be more theatrical and say,
dī tē/vōs salvent 'may the gods keep you safe!', which is often used as a deprecatory formula or a type of thank-you. Sort of like (Southern) English, 'Well, (God) bless your heart/soul!'
You could also say
heus!, which is like 'ho, there!'. Or
quid novī, frāter/soror? 'What's new, brother/sister?' (lit. 'what [is] of a new [thing]?')
