What makes group practice so powerful? According to H.H Jigme Phuntsok Rinpoche's teaching, the effect of group practice is far more superior than one practicing alone. First of all, the participants of massive Dharma gatherings can receive immeasurable merits, it was elaborated in scriptures such as "Stanzas of defining Dharma's Nature". Basically your mind is similar to a lamp, an individual's power is too weak to dispel the darkness at night, however when many lamps are put together, we can enhance each other's strength and light up the entire world.
Secondly, there are big differences in merits if you practice with or without preserving precepts, regardless if you hold lay practitioner's precepts, Bodhisattva's precepts, or the Vajrayana's Samaya precepts. In many Mahayana scriptures they say: if you have Bodhicitta aroused, then even if you recite a mantra for a single time, it will still generate huge merits vast as the Universe. When we participate in massive group practices together with practitioners who preserve the three precepts above, because of their precept holding, our practices will cultivate unimaginable merits.
Furthermore, many attain masters often participate in group practices as well, their blessings can easily bring forth our attainments. In the "Daily Recitation Prayers" of Chinese Buddhism it says: "If one practice together with Dharma friends diligently, there won't be any difficulties to attain Buddhahood and realization." By practicing together, we can reach Buddhahood swiftly and easily.
Therefore, we need to clearly understand the merits of group practices and cherish these precious opportunities. Lay practitioners are busy with daily life, hence they should join more group practices, from where they can get positive changes to their mind, as well as benefiting self and others.