At any given time, you can benefit from:
unlimited Prayer Auras
1 Cleansing Aura
1 Meditation Aura.
This means if you party with 7 Pallies running Prayer, you get the benefit from all 7 Prayers. If two of those Pallies switch to Cleansing, and another two switch to Meditation, you will get the benefit of THREE (all of them) prayers, ONE cleansing, and ONE meditation, with the extra Cleansing and Meditation being wasted.
If you're running Cleansing and you hire a Prayer Merc with Insight, you will get:
Healing from your merc's Prayer aura
Healing from your merc's Meditation aura
Healing from your own Cleansing aura.
Meanwhile, your merc will only benefit from his own Prayer and your Cleansing, since Mercs are always unaffected by Meditation.