This is the common Quenya verb for “to sit” (UT/305, 317), also used to mean “stay” (PE17/162) or “remain” (QL/39). An elf might say in greeting hara máriessë, but this means “stay in happiness” rather than being a literal invitation to sit down. The verb har- was derived from the root √KHAD “sit” (PE18/95; PE22/148). When describing buildings (and shorter hills) in Quenya, they are said to “sit” rather than “stand” unless they are particularly tall (PE22/125).
Conceptual Development: ᴱQ. har- dates all the way back to the Qenya Lexicon of the 1910s, where it was glossed “remain” under the early root ᴱ√HAŘA [HAÐA] “cleave [to], remain” (QL/39). The Qenya Lexicon had ᴱQ. soro- as the verb for “sit” under the early root ᴱ√SORO [ÐORO] (QL/85). The English-Qenya Dictionary (EQG) of the 1920s also had soro “sit” (PE15/77), and the Early Qenya Grammar (EQG) from this period had the past form sórie “sat” (PE14/46, 78).
The verb ᴹQ. har- reappeared with the gloss “sit” in The Etymologies of the 1930s under the root ᴹ√KHAD of the same meaning, but this verb and its root were revised to ᴹQ. ham- and ᴹ√KHAM (Ety/KHAM; EtyAC/KHAM). At some point Tolkien introduced a new root ᴹ√KHAM “call to, summon, name by name”, writing “KHAM sit (replacing KHAD, cancelled)”, which apparently meant ᴹ√KHAD “sit” was restored.
The Quenya Verbal System (QVS) of 1948 had both har- and ham-, but in that document ham- meant “sit” and har- meant “dwell, abide, reside” (PE22/125). In that document Tolkien said “Q ham- ‘sit’ is used often of cities, towns, houses (and also of hills that are not very high): dwellings, buildings (except high towers) are not said to stand”. √KHAD “sit” appeared in the second version of Tengwesta Qenderinwa (TQ2) written around 1950 (PE18/95), and there are no signs of ham- “sit” thereafter, only har-.
Neo-Quenya: For purposes of Neo-Quenya, I would assume that Tolkien’s 1948 statements about the use of ham- “sit” for buildings also applies to har- “sit”. I would also ignore the 1948 use of har- to mean “dwell, abide, reside”, using Q. mar- for that purpose instead.
har- vb. "sit, stay", pl. present hárar in CO (i hárar "those who sit, those who are sitting"). Imperative hara in the phrase (hara) máriessë "(stay) in happiness" (PE17:162). According to VT45:20, har- "sit" is derived from a stem KHAD which Tolkien abandoned in the Etymologies, but since CO is later than Etym, he may seem to have restored KHAD. If so, the past tense of har- would be *handë.