Beginning improvisers may not have belonged to a jazz ensemble before*, so they will need guidance about their roles and relationships to others in the combo. Provide scaffolds, like chord voicings or suggested bass lines, so that novice players can focus on the task at hand: performing ensemble roles . Model and coach them through their roles.
The descriptions below keep it simple; obviously ensemble roles and relationships can deepen as players move out of novice territory.
HORNS: All of the wind instruments in the combo belong to the horn section. During the head of the tune, a horn player's primary role is to literally perform the head of the tune, or the melody. If there is more than one horn in a combo, they can all play the melody, or one or more can perform a harmony. During improvising, a horn player who is not soloing either listens to other soloists or plays background figures to support them.
GUITAR/PIANO: Guitar and piano players belong to the rhythm section. During the head of the tune, a guitar or piano player typically provides a chordal accompaniment that follows the harmonic form. Depending on who else is in the combo, though, and the choices your students make about arranging a lead sheet, a guitar or piano player can also perform the melody. During improvising, a guitar or piano player who is not soloing comps changes (a rhythmic style of chordal accompaniment) to support other soloists.
BASS: Bass players belong to the rhythm section. During the head of the tune, a bass player establishes the pulse through performing a walking bass line that follows the harmonic form. The bass player and drum set player work as a team to maintain the tempo throughout. During improvising, a bass player who is not soloing performs a walking bass line to support other soloists.
DRUM SET: Drummers belong to the rhythm section. During the head of the tune, a drum set player establishes the rhythmic feel of the tune (e.g., medium swing). The drummer and bass player work as a team to maintain the tempo throughout. Depending on the tune, the choices your students make about arranging a lead sheet, and the drummer's experience, the drummer may perform fills or ensemble hits to complement the melody. During improvising, a drum set player who is not soloing keeps time to support other soloists. A drummer who already keeps solid time can outline the form, too (e.g., fill at the end of each section, create contrast in the B section of a tune). More experienced drummers will comp behind soloists and contribute more of their own musical ideas to support and complement the soloist.
* Some beginning improvisers may already belong to another jazz ensemble, like a big band. They have likely performed similar roles to those in a combo, but not in the same way. The primary differences for most of these players will be performing their roles independently, and from a lead sheet. For example, horn players will not have background figures written into their parts, they will create them from chord symbols and by ear. Similarly, piano players will not have fully notated comping patterns like in many arrangements; they will interpret chord symbols, choose rhythmic comping patterns, and follow the form.