You get 5% additive damage for each ally affected by your mantra. Allies include your Follower, your Mystic Ally, other players and their pet. When using Inna's Mantra (4) Bonus you get 3 stacks for each affected ally, one stack per mantra (Mantra of Conviction doesn't affect allies). This bonus is can stack up to 4 times for a 20% damage increase.
Additionally all allies affected by one of your mantras get their damage increased by 5% additively.
This passive is mainly used in groups to boost our teammates damage by 5%. The increased damage bonus we get can be better attained through other passives like Momentum. Note that if you're playing in the 4 Player party, you will only get +15% increased damage (3 allys x 5) out of the maximum 20% increase. In order to get the full 20% someone in the party will need a pet or minion that's impacted by our mantra.