writeth up. enrolleth.
people. peoples.
That. Instead of "That", supply "[and say] this one", &c.
Selah. Connecting the last repetition of the alternation, and completing the Structure. Thus, both the Selahs in this Psalm are structural.
Related Commentaries of Psalms 87:6
Psalms 87:6
6 The LORD shall count, when he writeth up the people, that this man was born there. Selah.