LIFX bulbs act as a device on your existing Wi-Fi network. The security of each bulb is as strong as the security of your Wi-Fi network.
If someone has access to your Wi-Fi network, and the LIFX app on their device, they will have access to control your LIFX lights. This is only while their device is connected to the same Wi-Fi network as the lights. When they leave your house, or no longer have access to the network, they no longer have controls to your bulbs. Giving your guests access to your Guest Network is recommended for security.
We recommend that you make sure your Wi-Fi network is secure. Use WPA or WPA2 security, and never share your Wi-Fi details with anyone you don't trust.