I keep seeing this at the end of sentences lately, but couldn't find much explaining it online. When I see it, I feel like it's usually serving as some kind of explanation but I'm not entirely sure. For example, I was watching a show and a character asks why the other is wearing a uniform. They then say: 今日バスケ部の試合だったんだ。ちょっとお手伝いでね。 I understand that he was explaining he was just there to help out, but I don't understand the nuance でね adds to the sentence.
This is not some special construction, but is a simple combination of で and ね. ね is a sentence-final particle used to seek agreement or draw the listener's attention.
で can be either a case particle or the continuative form of the copula だ. As a case particle, で has many roles, so its translation can vary.
(That is) in Tokyo, you know.
Within three days, okay?
With/Using scissors, right?
(You do it) alone!
Likewise, で in お手伝いで is a case particle that marks a condition/scope/situation.
I went to a basketball game as an aid/help.
As a little help, you know.
で can also be the te-form of the copula (だ). A te-form can be at the end of a sentence in casual speech. For examples of this type of でね, see: What exactly is this でね construction?