Communication between the switches may be affected briefly, while the Spanning Tree converges. One thing to be aware of is the risk of losing VLAN data if the switch you are adding has a higher VTP configuration revision.
If you add a switch with a higher VTP configuration revision number, it can overwrite / erase all of your VLAN information for the entire VTP domain.
Adding a VTP Client to a VTP Domain:
http://www.cisco.com/en/US/products/hw/switches/ps646/prod_release_note09186a00800aa034.html#66902