OK, here's a go at it.
A "workgroup" in Office terms is a group of users sharing files, one of which might be an Access DB.
The "workgroup file" is the file that contains the list of users allowed to share the files associated with that workgroup. It also describes the permissions for each user who could touch the affected files.
A user has an account in the workgroup file. This account includes the user's login name which can (but does not have to) match their network login name. The workgroup file also includes a password that can (but probably will not) match the network password. There is a numeric ID that you assign arbitrarily.
A workgroup file contains groups. If the group is being used correctly, it represents a role that a person can play. For instance, you could have a group called "DataEntry" - the group of persons authorized to enter data into a database. It could have a "Maintainer" group - the group of persons authorized to modify database objects. Or the groups could be "Structure Maintainer" and "FormReportMaint" - split responsibilities. You, as the implied security manager, get to lay this out as you choose. Each GROUP should have rights assigned. Don't assign individual rights to any user.
A workgroup includes two groups (Admins, Users) and one user (Admin) by default. The groups cannot be removed but their properties can be changed. I think you could remove the Admin user but I don't think you SHOULD remove it. Instead, you would make it a member of the Users group (it is anyway) but NOT a member of the Admins group. Be sure you have another account that is a member of the Admins group BEFORE you do this.
If you do the right thing in your design, you have pre-analyzed the roles you want various people to play. You have created new groups corresponding to each role. You have assigned permissions to each role for every object in the database. You have also created accounts for each person allowed to log in to the database. You have created users and assigned them to groups that correspond to their role within your organization.
Search this forum for specific advice on how to modify the permissions on the Users group and the Admin account to improve your DB security.