I understand the difference this way. Although both sentences indicate an action that will take place in the future, there is a slightly different point of view in the mind of the speaker. From a grammatical point of view, the first sentence (I’m going with my friends) seems to stress the present, that the decision has been made, that I have decided to go with my friends. The second sentence (I’m going to go with my friends) stresses the future…I intend to go with my friends. “Going to” is frequently substituted for “will” or “intend to” in spoken English. The distinction is slight and there is no difference in meaning.