Getting an agreement signed before tenancy when not all tenants are in the UK until after the tenanc

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I wonder if you can help. We met prospective tenants (students) informally a few weeks back and now going to go ahead with a tenancy, however, one is out of the country, and the others are dotted around the UK. The tenancy is due to start at the beginning of september, however, only two of them will be around by that date, the others actually moving in a week or two later. What is the best way of getting the agreement signed before the start of the tenancy? If the contract is mailed around from person to person, each putting a signature on it (with seperate witness on different dates) before it comes back here for me to sign, is that acceptable? Or is there a better way? Any advice much appreciated.

Thanks