Here's what I would do:
Create a process (call it Proc1 for this example). It should have inputs of EmployeesTbl AND ActivitiesTbl and have a rule saying :
EmployeesTbl.EmpActivityCap=EmployeesTbl.EmpActivityCap+' '+ActivitiesTbl.ActivityName
After that, add this as an Update Rule for EmployeeTbl:
IF ActivitiesTbl WAS ADDED TO EmployeeTbl.EmpActivities THEN
EmployeeTbl.EmpActivityCap=UNDEFINED
FIND ActivitiesTbl WHERE ActivitiesTbl IN EmployeeTbl.EmpActivity
Proc1
That should work I think...
PS. You could probably rather do this in the update rules, which would update the text field if an item is added OR removed from your multi selector list:
IF EmployeeTbl.EmpActivities WAS CHANGED THEN
EmployeeTbl.EmpActivityCap=UNDEFINED
FIND ActivitiesTbl WHERE ActivitiesTbl IN EmployeeTbl.EmpActivity
Proc1