Hello,
If I select more than one user, will it be necessary to both approve / reject or only one approve / reject?
If you select multiple approvers, only one of them will need to approve the request. There is no need for all the approvers to process the request.
Is it possible to put a group? Or does it need to be individually?
It is possible to use both approaches. You can specify groups or user accounts in the list of approvers. If you specify a group, members of the group will be approvers.