I can think of 2 ways to do this:
1) Have a summon ability for each individual combatant.
That should be pretty straight forward :)
2) Use a dialogue in the summon ability to select the combatant.
This largely depends on where the combatants are coming from, but you'll most likely use selected data to store the combatants (e.g. from the user's non-battle group). Using selected data allows you to use the Selected Data Choice dialogue to select one of the combatants, which is stored in a different selected data for you to use further (e.g. add to the battle).