From Guild Wars Wiki
From the player's point of view, an ally is any creature who may be target of non-offensive skills and can't be attacked. An homonym term refers to players that are members of guilds in the same Factions alliance as the player.
 Friendly creatures
- Each player is his or her own ally. (Skills which specify "other ally" can't be used on self.)
- All party members are also allies.
- Animal companions brought by party members are allies, but not party members.
- Summons, such as minions and spirits, will be allied to the party of the summoner and hostile to their foes.
- Allies who are not party members will not be affected by spells or skills that specify "party members" instead of "allies". For some skills this isn't true, but such cases are believed to be bugs in the skills' wording.
- AI-controlled allies will attack creatures they are hostile to, and heal and support other creatures they are allied to, but this may not be the same creatures that the party is hostile and allied too. For instance, they may heal both the players and the foes the players are fighting.
- Some non party members, such as some mission specific NPCs, will appear in your party window, below all other party members in the "Allies" section.
- In PvE instances, allies are shown as green dots on the radar and/or with green names in the user interface in PvE, There are two shades of green:
- Green is usually used for creatures that may appear in the party panel, and for some of the creatures that will not appear in the party panel, but follow and help the party.
- Lime is usually used for all other allies.
- For animals, green is usually used for hostile yet passive charmable animals, and lime for those animals that spawn as allies and won't become foes even when being charmed.
- In PvP instances, ally names and radar dots appear with the same color as your own, and foes in different colors.
- In all outposts other than Maguuma Stade, green is used for Henchmen and lime for all other non-hostile NPCs.
- The foe/ally status is not definitely fixed. In some situations, foes may become allies and vice-versa. Examples include ambushes in which NPCs may be friendly before attacking; minions becoming hostile when their master dies; or NPCs surrendering after being defeated.
- All creatures that are not allies are foes instead.
- While they are passive and their names are colored green, charmable animals are considered foes for skills that target creatures and all Point blank area of effect skills that affect foes, but allies for the 'select nearest ally' key command and some point blank effects that affect allies. If attacked or affected by hostile spells they will turn hostile, at which point they completely become foes.