Site Permissions with Groups

It would be a lot easier if I could had people to custom groups and just assign groups. There are people that need only access to one site. I have many tech that i would like to include in a group to give full access to all sites. There are many sales and account people i would like to give read only access to everything. If I could use groups at the site level to include all uses in the group it would make life a lot easier to manager when adding additional customers instead of having to manually select everyone's individual permissions.