"All of my friends like dancing."
会 has two meanings. 1) 会="can"/"be able to" for learned skills. In this case it is the main verb of the sentence.
2) 会 means "will" indicating future tense, especially for events that will happen objectively in the future. (For events that you are going to make happen through your own effort, it is more appropriate to use 要). In this case it is a "helping verb" coming before the main verb of the sentence.
The original sentence says "all" your friends and 都 means "all/both." 朋友 can either be plural or singular friend(s) based on context and does not ever really need anything extra to show it is plural. By saying "all" it is already implied to be plural anyway. Hope this helps.