Use Combined ritual magic rules- I use concentration check, with a synergy bonus from spellcraft and knowledge(arcana), instead of requiring a new ritual casting skill.
Using the combined ritual magic rules, you add 1/3 caster levels of participants with the same class, +1/4 caster levels with the same TYPE (arcane or divine), +1/5 caster levels of those with different type, +1 per participant who can actually cast the spell.
Time: for a standard action spell, 3 min x spell level x participants.
Cost: 10 gp x spell level x participants.
Assuming all your casters participate:
Peja - leader, cleric 14
Garrel - cleric 11
Doorgath - other divine, 14
Ari - arcane 13
Davreya - arcane 8
Bonus caster levels = 11 (must be distributed among all possible effects; I assume you'll have more than enough to cover Reisha)
Ritual time: 3 minutes x 5 participants x 7th level = 105 minutes. > 1 hour, so 2 ritual casting checks will be needed.
Cost: 10 gp x 5 x 7 = 350 gp. I would assume the components could be had in town.