Heat grill to medium-low (about 325°). Cut potatoes in half lengthwise, then cut each half into 3 long wedges. Put wedges on 2 rimmed baking sheets; drizzle with 1/3 cup oil, turning to coat. Sprinkle with rosemary, salt, and pepper.
Grill sweet potatoes covered (in batches if needed), turning every 10 minutes or so, until tender and browned, 15 to 30 minutes. Transfer to a platter.
Drizzle with a little more oil just before serving.
Make ahead: Up to 1 day, chilled. To reheat sweet potatoes on the grill used for turkey, adjust fire for direct heat: For charcoal, spread charcoal over firegrate and add 12 more briquets. Set potatoes on cooking grate; cook, turning occasionally, until hot, 6 to 8 minutes. For gas, turn on all burners to medium-low (about 325°).
Note: Nutritional analysis is per serving.