If you know for a fact that you want every login that is granted access to the SQL Server to access this database and that's not going to change (or you'll have time to make a fix when it has to change), here's the simplest solution:
- Create a role in that database that has the permissions needed by all
- Enable the guest user in that database
- Make the guest user a member of the role you have created
The guest user will be the user a login will use if they don't already have access to the database. So if you've explicitly granted access to other logins, you'll want to make sure they are members of that role as well.
The reason I'm not recommending that you give permission to public is in the event that you did have to manage a change in permissions, you could easily create another role, grant the specific login access to the database, and make it a member of that role.
K. Brian Kelley