Gurus of many organisations across the globe nowadays initiate many
individuals collectively in a hall by the same beeja mantra
If this is the case then every person being "initiated" know what the other persons are initiated into. And, that is not right.
Because only the Guru and the disciple should know which is the initiated Mantra. So, this process is flawed by "Mantra PrakAsha Dosha" - which is that the Mantra you are initiated into is also known to others.
Gurum prakAshayed dhimAn mantram yatnen gopayet |
prakAshAbhyAm nasyatah sampadAyushi ||
The Guru must always be revealed by the wise, but the mantra must
carefully be kept secret. By not doing these two acts - that is, by
not revealing the Guru and by not concealing the mantra, decrease,
respectively, [the disciple's] wealth and longevity.
Kularnava Tantram 11.88
Besides that, the Mantra should be spoken into the right or left ear as prescribed. So it is a kind of whisper into the ear and not a loud shout which anybody around can hear. Some scriptures recommend that at that time the eyes of the disciple should be tied with a piece of cloth.
From Rudra YAmala Tantram (quoted in Brihat TantrasAra):
Gurustu PrAngukhobhutva ShishyA Prachi Mukha Stithah |
Dakshine Karne VAme chaiva TathA Sakrit ||
Viparitam Tato Geyam
StreesudrAnancha VAmatah |
Guru should face the east and the disciple the west direction. Guru
should then speak the mantra into disciple's right ear 3 times and the
left ear 1 time. For Sudras and the women the process will be
So, technically this is not a DikshA at all and it does not have the approval of the Tantras.
Very often, these mantras seem constructed and so have no specific
rishi, devata and chhanda.
In that case the Mantra is not even a Mantra so what one is doing with it should not be anybody's concern because such a transfer of Mantra does not really mean anything.
Actually Guru should mention the various limbs of the Mantra too during DikshA.
Tatah RishiAdisamyuktam Mantram Gururdakshina karne Trih SrAvayitvA
VAma karne Sakrit SrAvayet.
Thereafter the Guru should speak the mantra, along with Rishi, Devata
and other Nyasas, 3 times in the disciple's right ear and 1 time in
the left (vAma karne Sakrit).
Whenever the process of initiation is described in scriptures it's only about one Guru and one disciple and not many. So, it is a one-to-one process.