I would Like to know the common elements between Group-1 and Group-2 so that the common Elements can be moved o new Group-3.
Any Help please
Go to Group > Operations > Booleans > "In All" and select groups 1 and 2.
This will create a new group containing all entities in both groups.
If you will work with three groups as you mentionend so you can create a fourth group using "Only in One" then activate the show mulitiple groups / show your three groups and hide the fourth one.
Next you will be able to see only the common elements for the three groups.
Give it a try and see if does what you want.
PS: Femap does not export groups in a .dat file. Make a neutral.