Creatures you control get +1/+1 and gain lifelink until end of turn. Untap those creatures. (Damage dealt by those creatures also causes you to gain that much life.)
The set of creatures affected by Tenacity is determined as the spell resolves. Creatures you begin to control later in the turn won't get +1/+1 or gain lifelink.
2016-04-08 • wotc
Multiple instances of lifelink on the same creature are redundant.