"They sit"

[BwanaSimba]

wamekaa

[juryrigging]

They have sat?

[BwanaSimba]

Kukaa can be used with different meanings:
a. be seated, as in amekaa, he is sitting.
If you are referring to present time you must use the -me tense because kukaa, with this meaning, is one of the verbs of state like kulala, etc.
wanakaa = they are in the process of sitting
wamekaa = they have finished the process of sitting, i.e. they are sitting.
b. stay or live somewhere as in nitakaa Mtwara.
In the present tense you would use -na-:
Wanakaa Mombasa, they are living in Mombasa