Multi select Text Functions

I think your best option here might be to have multiple multi select fields for each location and then use a formula to combine them all into one result when viewing a record

I do not recommend using a multi select field for this purpose as you will be very limited in your future reporting, for example, to be able to report on which employees attended, which meetings.   

A better solution is to set up how many to many relationship where one meeting has many meeting attendances and one employee has many Meeting attendances.

Then, on the employee record, you will have an embedded report of their meeting, attendances and of course, on the meeting record, you have a meeting of all the meeting attendances is as well, (the employees who attended)  

If you find that adding the employees is too slow there are faster ways to add the employees, but that would require a one-on-one consulting session with me.