Can a creature with summoning sickness be targeted?

Question

Can I target a creature as soon as it is played, even with summoning sickness?

Example: If my card, Beguiler of Wills, reads "Gain control of target creature," or Dungeon Geist's "tap target creature opponent controls," can I gain control or tap a creature my opponent just played? I have a friend arguing I can't, because they're in summoning sickness, but I thought that just stopped their tap-ability and attacking for the first turn.

Answer 1

Yes you can target a creature with Summoning Sickness. Summoning Sickness only prevents creatures that you haven't controlled since the start of your turn from attacking or using activated abilities with {T} (Tap) or {Q} (Untap) in their activation cost.

*Summoning Sickness Rule *

Informal term for a player’s inability to attack with a creature or to activate its abilities that include the tap symbol or the untap symbol unless the creature has been under that player’s control since the beginning of that player’s most recent turn. See rule 302.6. See also Haste.

302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. A creature can’t attack unless it has been under its controller’s control continuously since his or her most recent turn began. This rule is informally called the “summoning sickness” rule.

107.6. The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. See rule 302.6.

Answer 2

You are right. The only thing that summoning sickness prevents is using the creature's own tap ability or attacking. You are allowed to target a just-summoned creature with spells and abilities. You're also allowed to enchant it, to sacrifice it, to attach Equipment to it, and to block with it. You can even tap it if the tap ability belongs to a different spell or ability, such as if you have an ability that includes in its cost, "Tap 2 untapped creatures that you control."