When you're using group enrolment keys users are normally given a prompt if they enter the wrong enrolment key on attemptng to enter a course. The issue is that the enrolment key prompt refers to the course enrolment key not a group enrolment key. This can cause confusion and a fair amount of frustration too.

If you're not using course enrolment keys, or you are and can live without a prompt, you may find it useful to disable the enrolment key prompt option at:

Site Administration > Course > Enrolments > Internal Enrolment - Edit.

Set "enrol_manual_showhint" to "No".

(You'll need to be logged in a as an administrator to change this).