In your AD create a group like "Site_DB_Auth" as an example.
Ad all the members to this AD group.
Add this group, and assign a role like "public" or whatever access you need them to have.
Map this Login Group to a DB
ok...lets start...follow step by step:
1. In SSMS, under Security,Logins,click "New Login".
2. Left of the "Login Name" text field, click "Search"
3. Change the "Location" to your domain, as it defaults to the local server.
4 In the "Enter the object nme to Select" field, type your group name eg. "Site_DB_Auth"
and click "Check Names".
It should resolve with an underlining.
5. Click "OK"
6. At the Bottom of the screen, select the drop down "Default Database" and select the DB this group needs to access by default.
7. Ok, now on the left menu, 2nd option is: "Server Roles"
Tick the appropriate boxes to assign the roles.
8. Ok, now the 3rd option "User Mapping", tick the box next to the DB you want this group to have access to.
Also below the DB list, is "Role Membership" - confirm the correct "db_*" access has been selected.
9. You should be done for a start.
10. Click OK
In your DB list on the left in SSMS, drop down your DB List, then the DB used, drop down Security, then drop down "Users" - check to see if your new AD group is in the list.
If I understood your query correct, then this should resolve the Authentication issue.