What to do when taking photos with a women??

Hand on the shoulder or upper back for friends. Waist or lower back if youre together.

This.
Just to add, when friends /family hug each other, you may notice that they tend to lean in and put their arms around the neck or shoulders; keeping a "safe" distance between the lower torsos and groin areas. This tends to happen regardless of the sex of either person. However, when couples embrace, their lower areas are far more likely to be in direct contact with each others (ditto for people who like each other). For a romantic, they are hands may be on the lower back and they lean back (unless they're about to snog or something). Hand on the ass suggests more of a direct sexual interest. Think about it, if one of your guy-mates hugged you in a way in-keeping with my latter description, I'm sure it would make you uncomfortable (assuming you're completely straight obvs).
I agree the comments from your friends were immature... although given your respective ages, it's not entirely surprising, so I wouldn't necessarily give them a hard time about it. If it does bother either of you, then just set the record straight (for the record lol)

Thank you for the advice, I've already set this issue straight before but me and her just both ignore it because its bound to continue anyway.
Thank you everyone else for your comments too