All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we say that the hadeeth that reads, "If the bearer of the Quran acts upon it, deeming what it makes lawful as lawful and what it makes unlawful as unlawful, he will intercede for ten of his family members on the Day of Resurrection, all of whom deserved to enter Hellfire," was narrated by Al-Bayhaqi and At-Tabaraani, and Al-Haythami said that its chain of narration included Ja‘far ibn Al-Haarith, who was declared a weak reporter by Hadeeth scholars. [Majma‘ Az-Zawaa'id]
The hadeeth was classified by Al-Albaani as weak in his book Dha‘eef Al-Jaami‘, no. 10133.
Even if one was to assume that the hadeeth was authentic, you cannot promise anyone to intercede with Allah for him on the Day of Judgment because you cannot be certain that you will be among the bearers of the Quran which the hadeeth refers to in the first place. In fact, such a promise implies self-praise and being certain that you shall be among the bearers of the Quran. Indeed, the person's deeds are judged according to the concluding and final ones; you do not know what your final deeds in this worldly life will be.
Even if you said, 'If Allah admits me into Paradise (I will intercede for you),' you still cannot promise anyone that you shall intercede for him because none has the ability to intercede with Allah on the Day of Judgment except those whom Allah shall grant permission to intercede. It is possible that Allah does not grant you the permission to intercede for those whom you promised intercession, and it is also possible that these people will not be worthy of the intercession in the sight of Allah. Even the angels cannot intercede with Allah except for those whom He approves of and by His permission.
Finally, you cannot promise others intercession in the worldly life as if you are positive about being granted the permission to intercede! So, fear Allah, The Almighty, and refrain from speaking of matters of which you have no knowledge.
Allah knows best.