They are both the same but conjugated for different pronoun. Ele/ela/isso/você TEM (he/she/it/you has/have), Eles/elas/vocês TÊM (they/you have)
I think in English you have to use the plural form, otherwise they are the joint owners of one shirt…
Is it the case that there are multiple shirts in the Portuguese answer "Eles tem (with the circumflex I haven't got on my keyboard) uma camisa"? There is a similar construction in French ("Ils portent leur chemise") where it is understood that they have one shirt each.