flag grouping

[Danny Sosa]

Hi! I'm new to lilypond and I have my first doubt about the using it.
The tutorial has been very helpful and I've been able to do most of my first project without problems.
What I am unable to do so far is change the grouping of flags.
in 6/8 time for example, is there a way to change the grouping of 8th notes from two groups of three to three groups of two? how about one big group of six?
The piece I am trying to typeset at the moment starts with a 5 note scale pattern as an anacrusis on the soprano line:

\relative c'' {
  \time 6/8
  \partial 8*5
  \key e \minor
  \stemUp
  b8 c d e fis |
}

The original score has that pickup as a big group of five 8th notes, and at often times has full bars as one big group of six.
By default, lilypond sorts the notes into two groups, one of two 8th notes and one of three 8th notes. Please teach me how to change that.
Thank you very much for your time!