I think this is a pretty good explanation: https://askaboutgerman.wordpress.com/2013/03/07/difference-jetzt-nun-german/
I think it is because there is no comma after "nun". From what I understand, "nun" means "well" as an interjection, but otherwise it means "now", so, "Well, I can join in," would probably be, "Nun, ich kann mitmachen," whereas, "Now I can join in" is "Nun kann ich mitmachen".
As already mentioned above, I think this is a good explanation: https://askaboutgerman.wordpress.com/2013/03/07/difference-jetzt-nun-german/
But all those suggested EN expressions can be used not only for actively doing something together, but also for a "passive" participation (like watching a movie, going to a concert, attending a party...), where I feel mitmachen would be out of place. Questions: (1) Am I right about mitmachen? (2) Is there an EN expression to indicate an "active, working" involvement? IF the answers are Yes and No, respectively, THEN cooperate, work together etc. should be accepted as well. (The translation gap is the same width, only in the opposite direction.) Appendix - in case "work together" would be acceptable as an approx. equivalent: I'm not sure whether "Now I can work together" is an acceptable stand-alone sentence in EN. If not, then additions of "with you/him/her/it(institution)/them" should be accepted as well.
It's a statement. I'm guess what's confusing you is that the subject is after the verb. That is done because "nun" was placed before the verb for emphasis, and the verb is in a fixed position, so the subject gets displaced. To actually form a question, you would start the sentence with "können" or use a question word. E.g.
- Kann ich nun mitmachen?
The normal word order of this sentence is: Ich kann nun mitmachen. If you want to emphasize "now" because until now you could not, then you may put "nun" in the first place of the sentence. The second place is reserved for the modal verb "kann" and the subject "ich" moves into the third position.