The Dunedain Rangers function similarly to their long-lost kin in Gondor, the Ithilien Rangers. They are a cheaper but not as good alternative to the Rivendell Archers and are recruited from the Dunedain Outpost.
The Dunedain aren't upgraded like the elves are, and the upgrades researched at the Imladris Forge do not apply to them. To upgrade the Dunedain with a Banner Carrier, a level 5 Halbarad must be nearby to the battalion. To upgrade the Dunedain with Fire Arrows, a level 7 Halbarad must be nearby to the battalion.
The Dunedain Rangers of Imladris come in a battalion size of 10 archers. Their recruitment cost can be lowered by building Hobbit Farms on settlements. They are not as powerful as Imladris' elven units, so by themselves, they won't last very long in a fight. However, because of their low cost, it is worth it to invest in a Dunedain Outpost to produce cheaper archers, especially if the Barracks is still level 1 and Rivendell Archers are not yet available. To increase their durability, it is essential to recruit Halbarad as well and level him up quickly; once he is level 7, the Rangers and the other Dunedain units will be able to purchase all of their upgrades. He also offers them a leadership bonus, so it could be a good idea to keep them close to him. In addition to the leadership bonus, Halbarad's level 10 ability (only available with his bow) gives them a +33% attack speed +20% movement speed. The Rangers can make use of their Stealth and Unexpected Salvo abilities to ambush enemies and deal high damage before they can react.