The government stipulates that groups must set aside £4,500 per adult. So, if you were to sponsor a family with two parents your group would have to show that it had £9,000 in a bank account ringfenced solely for the purpose of sponsoring. Depending on the part of the country you were from, and the exact arrangements for sponsorship you enter into, you might need to raise more money than that. On the other hand, you could find that this money is not all spent. A lot depends on the needs and progress of the refugee family you sponsor.
More details in the fundraising plan section on the Plan page.