The Oracle: Sharing the wealth

A – There are two aspects to your question. The first is your entitlement (or otherwise) to a share of the advance, based on the fact that you participated in making the demo. The second is your potential entitlements as a songwriter, if you are one.
First your involvement in the demo: is there a band agreement in place? If so, look to see if this contains any provisions relevant to your question. If not, realistically, it will be very difficult for you to sustain an argument that you are entitled to a share of the band’s recording advance. The reason? The advance will be paid under a recording agreement entered into between the record company and each individual member of the band, i.e, you will not be a party to this agreement. The other band members might voluntarily agree to give you a share of the advance – but I presume not, as you’d hardly be writing to Hot Press!
Turning next to your entitlements as a songwriter, if you are one. I take it we are talking about original compositions here, and you say you “worked on the arrangements”. Whether your contribution amounted to you being a co-writer or co-lyricist of the underlying music is a question of fact which only you can answer. Did you assert copyright? If so, then while you may not be entitled to a share of the advance paid by the record company, you would certainly be entitled to a royalty based on the exploitation of those songs in which you hold a share of the copyright.
It is worth bearing in mind that you are leaving the band voluntarily. You could have chosen to stay in the band if you wished, signed the record deal, and participated in a share of the advance. Unless there was some agreement between the band members to the contrary, by walking away from the band you effectively gave up your right to participate in the record deal, and thereby the advance.