The laws will vary from place to place but I believe that a doctor will (sometimes, especially if he knows and trusts you) give you a prescription for some medications if you are taking a group of people on a back-country or remote country expedition.

As a first-aider (in Canada), I can administer prescription medications under the supervision of a doctor; this can include being in telephone or radio contact.

However, from talking to a couple of fellow first-aid volunteers who are severely allergic to insect stings (and are therefore much more knowledgeable about this than I am), almost no two people will react the same to epi-pens. The more often you've been stung, the less time you have to react and the less effective the epi-pen will be. One woman I know keeps her epi-pen clipped to the outside of her jacket (in such a way that if the clip breaks, it will fall inside the jacket and not on the ground) because she won't have time to fumble for it if she gets stung; she also makes sure that anyone she's travelling with knows how to use it, and she carries a letter inside the case that essentially waives any right to sue if someone gives her an epi-pen injection when she's unconscious - her condition is that serious. Another carries 4 epi-pens on the trail, because each one is only good for 15 minutes.

My understanding is that you can also administer medication to a member of your group if (1) you have a written letter of permission from that person (or their legal guardian, if applicable) and (2) have been specifically trained in administering that particular medication to that specific individual - preferably signed off by that individual's family doctor.