The simple principle here is TTE has to know about it.
Since you already have confirmed ticket and 2 from your group are already boarding from Lucknow itself. Just tell them to show their e-ticket to TTE with their ID Proof and notify the TTE that rest of the passengers will be boarding from Bareilly due to sudden change in plans.
Please ensure that those 2 passengers have pictures of ID proofs of other 4 passengers boarding from Bareilly.
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case your ticket is from PRS Counter, ensure that those 2 boarding from Lucknow have the original otherwise it'd be counted as traveling without ticket and entire ticket would be liable for cancellation by the time train reaches Bareilly.
TTE will not cancel that ticket since you've already represented your group by those 2 members.
In June I had a booking with my Colleague in Karnataka Sampark Kranti from Yeshwantpur to Kacheguda with boarding at Hubballi. Due to last minute changes we were stuck in Bellary and changing Boarding Point window got closed. Had my Colleague in Hubballi notify the TTE about change in Boarding by sharing pictures of our ID and PDF of our E-ticket. It worked and TTE waited for us till Bellary- it's an extreme measure do not try it as TTE might be different but the simple fact is TTE has to be notified truthfully.