The response from Barry Margolin was absolutely correct. There will be no interuption by adding another standby group router. Just to expand on it:
HSRP has the following states, listed in sequence: 1) Initial 2) Learn 3) Listen 4) Speak 5) Standby 6) Active
If an active HSRP router exists on the network, any joining router goes through the states and remains on standby. If the "preempt" option is specified, the highest priority router becomes the active router. If the "preempt" option is not set, the active router remains active no matter if another router has a higher priority or not. This promotion from standby to active without "preempt" only occurs when the standby router does not receive HRSP hello packets within the hold time and the standby router becomes the active router.
HSRP hello packets are UDP port 1985 multicast packets from the router interface IP address to 220.127.116.11 with at TTL of 1 as defined by RFC 2281.