I'm a singer and pianist who has worked with some proffessional accompanists and I've accompanied some singers before.
There is no official way to 'claim' yourself as an accompanist, it's like teaching you are basically self employed. However it helps to be part of an association or guild ... it's good to have some sort of qualification.
There are grade 8 pianists and A mus A pianists I know that can't even sight read. I am not kidding, they can't read well. To learn a piece fast and to accompany well it is essential to have good sightreading skills. A good accompanist also follows the soloist and doesn't bang on the thing but brings out the soloist. Also, a good accompanist can arrange and improvise or jump in if the soloist is doing anything wrong or forget's his/her bit. One accompanist I had, held the arrangement of something that played my melody, he arranged it so that the part didn't double. It's being able to understand some theory and being able to read well that makes accompanying an easier job.
Alright, I bet you already knew that. I think to really get into accompanying it's good to get as much experience as you can by doing free jobs in school, finding musicians (singers, flautists, violinists, etc). The more experience you get the more you understand instruments better. A flautist or singer can only hold a note for so long until they die of no oxygen. A saxophonist often needs the music to be transposed into Bb, etc.
Fortuanatly Trinity College London (also known as Trinity Guildhall) supply a piano accompanying syllabus from grade 5 upwards. Some teachers (who are also accompanists) I know occasionally coach an ensemble on some lessons.
An exceptional accompanist that I once worked with told me that she just started out playing in schools. She told me that she learned so much through that. I think that is the key, experience!
