Platinum can use the Diamond check in, it's across from the regular regular front desk. Two big doors, they have cookies on weekends and give you some bottled water nice touch.
Rio also has a Platinum check in area. It is part of the Diamond and Seven Stars check in. If you walk in the main front entrance regular check in is to the left. The Platinum/Diamond/Seven Stars is off to the right and has big glass doors (just pass the yogurt place and purse shop). Very speedy compared to regular check in and they did give us cold waters too.
Bally's Platinum is to the left of the main check in (closer to the valet entrance). I've never seen more than 3 people in the Platinum line compared with 30 plus or more some Fridays at Bally's!