Is saying "Peace be upon the Messenger of Allah" by the congregation in a gathering considered an innovation (bid'ah)?

The statement of one who says, "May Allah bless the Messenger of Allah," in a congregation is not an innovation (bid'ah). Rather, it is recommended (mustahabb), because sending blessings upon the Prophet, peace be upon him, is a recommended Sunnah, whether the person is alone or in a congregation. Reminding others of it is a reminder of good, and doing it frequently is a cause for relief from worries and forgiveness of sins, as stated in the hadith of Ubayy ibn Ka'b: "Then your worries will be relieved, and your sins will be forgiven."