Does Stand United give +1 to other units or just buff one unit and that's it?

Stand United does both.

When you play Stand United, it performs two distinct actions:

1. Buffs one friendly unit: It places a buff counter on one friendly unit (provided that unit does not already have one).
2. Creates a global passive effect: It creates a temporary, board-wide passive effect that causes all of your buffs (including the one just placed and any others already on the board) to provide an additional +1 Might for the rest of the turn.

In practice, this means that every friendly unit you control that has a buff counter will gain a total of +2 Might for the turn (the +1 from the buff itself, plus the +1 from the passive effect granted by Stand United).

According to FAQ 1457 and FAQ 851, while the targeting portion (placing the buff counter) is restricted to the battlefield where the spell is played, the global +1 Might effect is not. It applies to all your buffed units on all battlefields.