I was looking at Dark Foresight, and I don't see any restrictions on targeting like I do with the Charm invocation. Therefore, it occurred to me -- can a warlock therefore keep Dark Foresight going on the entire party at all times, and furthermore provide telepathic communication through that link between all party members (where the warlock is acting as the relay)? It seems very strong to me, but I cannot think of a reason why it would not work. In a party of 5 people, it would cast a huge net over an area giving the warlock an almost omniscient spider-sense of the situation.
The dark invocation dark foresight (Complete Arcane 133) doesn't have any restrictions on its use, saying only that the warlock "can use foresight as the spell" and that if the warlock is "within 100 feet of and [has] line of sight to the target of the ability, [the warlock] can communicate telepathically with the target." There's no limit to the number of targets on which the effect can be used simultaneously.
While being able to employ a 9th-level spell-like effect on anyone she can touch is powerful, a warlock is typically at least level 16 by the time she gets her first dark invocation. At level 16 the ability to have an effect like the spell foresight on anyone and communicate telepathically with those within 100 ft. on whom the effect's been used is interesting, but unlikely to break any games.
I think that is the intended use of the invocation. Insight bonuses of +2 AC and +2 Reflex isn't game breaking at 16th level IMO. Also, if you read Foresight, you'll note
"The subject, however, does not gain the insight bonus to AC and Reflex saves."
meaning only you, the Warlock, will get the bonuses, you'll just know when the others are in danger and keep them from being surprised or caught flat-footed.