API UnitPlayerOrPetInParty

Returns 1 if the unit is in your party group, nil otherwise.

isTrue = UnitPlayerOrPetInParty("unit")

Arguments

 * "unit" : String - unitId who should be checked

Returns

 * isTrue : 1 or nil

Note: returns nil for player and pet as of 3.0.2

Example
local TargetInParty = UnitPlayerOrPetInParty("target")

Result
TargetInParty = 1  - If your target is in your party TargetInParty = nil - If your target is not in party