The difference is that the first sentence has fewer words than the second sentence. The first is more colloquial, the second is more concise. In speech, we often leave out words or phrases when the meaning can be implied, or determined, from the context. "I am going (to go) with my friends!"

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.

if you want to know the grammar behind this future form, it works like this: "I am going with my friends tomorrow." (You need a future time for this) - this is an arranged plan. "I am going to go with my friends." - this is the future, as predicted from the present situation. This can work even if you have a plan in your head. If you say "I will go..." it is possible that you only have an intention, but not a plan.