The group, called Diné C.A.R.E., described the mine as an accident waiting to happen.
Three quarters of a million dollars was already approved last fall by the Navajo Nation Council to pay consulting companies to study the deal, and the council recent approved another $2.3 million for further study.
Little from that study has been revealed, including how the funds were spent, according to the group.
"One of the biggest concerns is that an environmental impact study has already been started," said Lori Goodman, of Diné C.A.R.E.
The study is being conducted by the U.S. Environmental Protection Agency and should be cause for concern because it is the first of its kind done on the mine, she said.
Mine operators have been dumping coal ash and other waste products.
It is unknown whether BHP Billiton will clean up the area before they relinquish control of the mine.
"The process will leave no stone unturned," Goodman said. "I think BHP is jumping the gun, given what the (potential) liabilities are."
BHP Billliton officials, in a Friday statement, said they have responded to questions from council delegates regarding the proposed purchase and that the additional study will allow Nation officials to make informed decisions.
But Goodman said information given to council delegates has been piecemealed.
"This agreement is being railroaded on the council," Goodman said. "This is a business decision being made by BHP, and so far they've pulled the wool over the Nation Leadership's eyes."
Navajo Nation officials could not be reached for comment on Friday.
Council Delegates and other Nation leaders are being told that there is no money to be made in renewable energy investment, she said, but there are areas on the reservation that have had success with solar power.
"Instead of investigating for themselves, they hear these reports ... our council delegates have their heads in the sand, and that's sad," she said.
The president of BHP Billiton New Mexico Coal said that demand for coal from the mine will drop 30 percent with the upcoming closure of three of five coal boilers at the Four Corners Power Plant, in a January Daily Times story.
The potential fall in the demand for coal is just another reason why the deal for the mine will be detrimental to the Navajo Nation and its citizens, Goodman said.
BHP Billiton released a statement Friday afternoon regarding the negotiations.
"New Mexico Coal Asset President, Mr. Pat Risner, appeared before the NaaBik'iyati' Committee of the Navajo Nation Council as part of the Nation's due diligence process for the sale of Navajo Mine," the statement said. "Mr. Risner appeared at the Committee meeting in the capacity as an expert witness' and at the request of the Council Delegate Lorenzo Bates, who is the sponsor of the legislation to fund phase two of the due diligence for this transaction. Mr. Risner was asked a number of questions about the transaction on topics including current status of the transaction, as well, as timelines and future of the Navajo Mine."
Funding for second phase of the process will allow the Navajo Nation to conduct a thorough evaluation of Navajo Mine and will allow the Navajo Nation to make an informed decision whether to purchase the mine, the statement said.