A bit more elaboration- The LACP configuration is defined prior to the mlag, via the 'enable sharing' command. When using LACP with MLAG, you have to define the LACP characteristics on the individual port(s) prior to adding them to the MLAG. In essence, you create a share using LACP on MLAG peer 1; you then create a share using LACP on MLAG peer 2; and finally you create the MLAG port between the two peers using the shared ports on which you defined LACP. See the example below. Port 1 is aggregated in a single load share for a 2 port MLAG, and ports 2/3 are also aggregated into a single share for a 4 port MLAG. I've left out the extra commands relating to the isc and mlag peering.
### MLAG PEER 1 CONFIG ###
# Module vlan configuration.
# this enables mlag to utilize lacp
enable sharing 1 grouping 1 lacp
enable sharing 2 grouping 2,3 algorithm address-based l2_l3 lacp
# Module vsm configuration.
enable mlag port 1 peer "CORE-02" id 1
enable mlag port 2 peer "CORE-02" id 2
---------------------------------
### MLAG PEER 2 CONFIG ###
# Module vlan configuration
# this enables mlag to utilize lacp.
enable sharing 1 grouping 1 lacp
enable sharing 2 grouping 2,3 algorithm address-based l2_l3 lacp
# Module vsm configuration.
enable mlag port 1 peer "CORE-01" id 1
enable mlag port 2 peer "CORE-01" id 2
Good luck, hope this helps to clarify!
Sean