In an organized ride situation, riders should expect to draft and to be drafted. It's the dynamics of a mass start and folks finding their pace, road and traffic conditions, etc. So yes, it's acceptable to draft for a while.

It's a good idea not to draft too tight until you assess that they are competent: I usually don't get closer than one bike length at first. It's also courteous to offer to pull if the situation warrants it. Communication never hurts.