The sentence [Let's eat separately"] if used in a group of 2 people, is clear in meaning and perfect in grammar, isn't it?
It means that if there's 2 people, Person_A will eat by himself and Person_B will eat by herself.
But if there's a bigger group, (say a group of 4), it's not clear whether [Let's eat separately] means that:
PersonA will eat alone, B will eat alone, C will eat alone, and D will eat alone
or if it means
PersonA eats with someone else or someones else.
So my question is:
With a group of 4 people (person A, B, C and D). How can you start this sentence [Let's eat ..... ] so that you end up with the meaning that:
PersonA will eat alone, B will eat alone, C will eat alone, and D will eat alone.
Can you say Let's all eat by himself?
Surely, you can't say, "Let's eat by ourselves", because that means somethingg different: A, B, C and D will eat as one group, but they won't eat with F, G, H, and Person_I.
In a group of four, if one person said to the whole group "Let's eat separately" would be a suggestion that each of the four should go off and eat on their own. It would be a crazy thing to say, however.
If, more probably, A turned to B and said "Let's eat separarely", then that would be an invitation for B to go and eat with A, but away from C and D. Perhaps they have something private to discuss, or perhaps they don't like C's and D's table manners.
You can't say "Let's all eat by himself". You can say "Let's all eat by ourselves" (to a group of more than two), but that's that same as "Let's eat separately.
Hope that's helpful.