So each employee record has a Group.
Your training table (each class?) should contain a field called Group. If the class is for many groups, you can use a checkbox of groups attached to the Group field.
Your relationship would then be:
Employees::Group = Training::Group
A portal on your Employee layout would display all the training classes (if they contain this employee's group in the Training::Group field). However, when you use a multi-line is this way, you lose the ability to generate Training reports sub-summarized by Group.
If you need the ability to easily generate reports then a join table (holding only the EmployeeID and TrainingID) would be required. But it seems you don't want to go that direction because it would again require you entering each employee each time they attend a training class.
UPDATE: It just doesn't feel right however ... if you hire a new Employee and decide that they belong to a certain group, then it would suddenly appear that they attended all the trainings when they didn't. So it depends upon the purpose of the Group field in the Employee table.