Starting out with your friends is a great way to 'learn' to effectively play mulitple roles in a group. Making mistakes in front of you friends and guildies shouldn't be that big of a deal as long as you have told them you are learning. Chances are you will pick it up quickly and eventually be quite effective for your friends. With hunter/priest I imagine your main focus will be healing, you can afford to be a little less effective on your DPS/CC role, especially if the rest of the group is effective in those roles.