Translation:I love you
It's very common, in everyday conversation, to omit the ni- personal prefix in present tense. So nakupenda instead of ninakupenda, nataka instead of ninataka, and so on.
Most people use the nina- form and the na- form interchangeably, but some would use the two in slightly different settings. The na- form would be the first person present indefinite form, when you are talking about something that is done right now, but it doesn't make clear whether the action is finished or if it is just finished (present perfect, nime- form) or if it is a process still in action (present continuous, nina- form).
It's in the notes for the skill "Object Infixes" (Obj. Inf. in short). For persons, the object infixes are as follows:
-ni- 1st p. sg.
-ku- 2nd p. sg.
-m-/-mw- 3rd p. sg.
-tu- 1st p. pl.
-wa- 2nd p. pl.
-wa- 3rd p. pl.
So if you want to say "I love her/him" it would become "Ninampenda".
And yes, both second person and third person plural use the same object infix.