What do you exactly mean by personally ? if you mean "Hi there, do you want to buy me a cup of coffee " or "Oh no
! MARS ROVER have found Aliens ", then surely I am not aware if mikrotik can perform this function
, better to use some chat application like VYPRESS, or other to send general or personalized messages.
I guess your clients should have some sort of chat type client installed on there pc. Net send command donot work with the newer version of windows, it was closed dueto various security reasons including the famous flooding attack of mass messages. Also you can set a little message on your AP's wifi network name title including your mobile number etc.