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Some countries have a formal invitation process that addresses maintenance and accommodation and some do not. For those that do not, it often boils down to how the person presents himself at the control point.
In a lot of cases they will take the person's word for it and a letter is simply a nice-to-have. The choice is yours, and in the absence of a formal letter you can draft one. If you do opt to draft a letter, make sure it includes...
and of course (where reasonable) be in the arrivals area and reachable by phone. The basic letter has variants, especially if the visitors include children or appear to be vulnerable in some other way. You didn't mention any such irregularities so we don't need to go in to them here.
In the absence of a stated requirement, the letter does not need to be notarised and should work fine without a wet signature.