A single attack that, if attack-granting is an ability that costs an action, consumes your ability to take an attack action and thus precludes your benefiting from the extra attack feature, or from taking bonus action attacks that require you to have taken the attack action. In other words, if granting an attack costs an action, you do so at the cost of all the attacks you'd ordinarily be able to make on your own turn, and the ally who makes it should likely have to spend a reaction to act off-turn. So you're getting one sneak attack enabled attack at the cost of all the attacks you could make and any other reactions your ally could make.
Also, you said 11d6+5. Presumably that's 10d6 from sneak attack, 1d6 from the weapon, and +5 from the rogue's ability modifier. In order for the rogue to have 10d6 sneak attack she must be like 19th level. Assuming the whole party is the same level, or within a level of each other, a warlord who gets extra attacks at the rate the fighter does would have 3 attacks per action by then. If the warlord were only interested in damage, let's say she picks a 2d6 damage weapon. So, that's 6d6+15 damage (three attacks each at 2d6+5). Using average die rolls (3.5 for each d6) that's only an 8 point difference, and that's without assuming GWM, Martial Adept or any other feats. It also assumes the conditions are right for the sneak attack class feature to apply to attacks against the target.