Two speakers, or one speaker and one answer. Verses 1 and 2 give three plural commands, בָּרְכוּ (bar-chu, v.1), שְׂאוּ (se-u, v.2) and וּבָרְכוּ (u-var-chu, v.2), all addressed to the servants standing in the house at night. Verse 3 replies with יְבָרֶכְךָ (ye-va-rech-cha, v.3), a verb whose suffix is singular, so the group is answered one at a time. The night is not decoration. הָעֹמְדִים (ha-om-dim, v.1) is a participle of standing, which is the ordinary word for being on duty in the sanctuary, and בַּלֵּילוֹת (ba-lei-lot, v.1) puts that duty in the plural of nights, meaning night after night rather than one occasion. Read straight, the psalm is a shift handover: the people leaving tell the people staying to keep blessing, and the people staying send a blessing back.
The crux is קֹדֶשׁ (ko-desh, v.2). The word is a bare noun, holiness or the holy place, with no preposition in front of it, so the line can be read as lift your hands toward the sanctuary or as lift your hands in holiness. The reading is genuinely disputed and the Hebrew does not decide it; older English versions take the first, and commentators who read the noun adverbially take the second. Note also that one root, b-r-k, carries the whole psalm: the servants bless in verse 1, bless again in verse 2, and are blessed in verse 3. The same verb runs in both directions, which raises the old question of what it can mean for a person to bless God, since nothing is added to Him by it.
The last three words of the psalm, עֹשֵׂה (o-se, v.3) to וָאָרֶץ (va-a-retz, v.3), are a fixed formula that appears four other times in the book, at 115:15, 121:2, 124:8 and 146:6. Two of those, 121 and 124, are inside this same collection of fifteen, so the group of Songs of Ascents closes on the phrase it has been using all along. The placement is pointed: a sequence of psalms about going up to Jerusalem ends not at a festival crowd but with a handful of people awake at night, and the blessing they receive is credited not to the Temple but to the maker of everything. The servants standing at night are usually identified with the priests and Levites of the watch; the psalm itself names no office, and the reading that extends it to anyone awake in the house of God is available from the plain text.
Night in the mystical reading is the condition of service without visible result, which is why this psalm and not a daytime one closes the ascent. The kabbalistic literature hears in the root of blessing the word for a pool, and reads a blessing as a drawing down of what has gathered above, so the two directions of the psalm are one circuit rather than an exchange of compliments: the servants raise their hands, and what comes back down through מִצִּיּוֹן (mi-tzi-yon, v.3), from Zion, is what they raised. The hands of יְדֵכֶם (ye-de-chem, v.2) matter for the same reason. Blessing is described as something done with the body in a specific place at a specific hour, not as a state of mind, and the psalm ends by naming the maker of heaven and earth to keep that local circuit attached to the whole of it.