1. Question: Why do friends of the Chosson put on tefillin now a days (from time of the Rema) throughout sheva brachos even though it’s possible they might get drunk and are then forbidden to don tefillin?
A. In the Mishna Berura (38:7) the Mechaber writes that a chosson and his friends who are joyous with him and all those connected to the chupa are exempt from tefillin because it’s possible that it will lead to drunkenness and lightheaded atmosphere.
B. The Chofetz Chaim in Mishna Berura (23) says that the Teshuvas Rema (132) poskins that now a days where even the chosson is required to say krias Shema and shemone esray (see siman 70), then automatically the chosson and anyone associated with the chupa are obligated in putting on tefillin. The Olas Tamid and Birkay Yosef poskin like this Rema.
C. Concept in halacha that if you are involved with one mitzvah you are exempt from the other. D. The Shulcham Aruch says 70:3 that if one marries a virgin, he is exempt from saying krias shema the first 3 days of marriage because he is too busy being involved with the mitzva of marriage. But that was originally, but now a days where people normally do not have proper intent in the davening any ways then even a chosson says the shema.
Answer: The level of fulfilling a mitzvah is lower now a days so just as the chosson cannot fulfill his mitzvah to the biggest capacity so too his friends don’t have the heter to drink since they are expected to fulfill their mitzvah to the highest capacity.