Plus Add does not add a group of students if one of those students has a conflict.

When Plus Adding a student who has a conflicting activity, the user will receive a system error message that states the student “has been plus added or is in a higher priority activity with Teacher X”.  

When Plus Adding multiple students, one student with a conflicting activity will result in an unsuccessful Plus Add for the entire group.  You will need to remove the student(s), with the conflicting activity, from the list and resubmit.  This can be done by clicking the “x” to the right of their name as shown below.