The aggregate interface inherits settings from the first physical interface added to the aggregation. 

When you add an IP address on the first member, it will be reflected on the aggregate interface.

This also applies to other common settings like:
• MTU size
• Reject inbound
• Allow intercept
• VLAN trunking
• Native VLAN
• Spanning tree
• IPv6 auto-linklocal

More information can be found in the SGOS 6.6 and 6.5 Administration Guide under "Improve Resiliency or Create a Bigger Pipe with an Aggregate Interface".