Characters
Some languages (for example, German) have two forms (masculine and feminine) for most words describing people. In German, feminine nouns usually carry the ‘-in’ suffix. The best way to address people would be to name both, while placing the feminine form first:
Liebe Mitarbeiterinnen und Mitarbeiter, […]
This, however, takes a lot of space which might be impractical for user interfaces. A solution would be to use a character to separate ‘-in(nen)’ from the masculine word stem.
Star
The gender star is being used a lot in German. However, it takes more space than "Binnen-I" or a colon. It also may be harder for the eyes to focus on the text with asterisks, since they are typically placed in superscript.
Liebe Mitarbeiter*innen, […]
Binnen-I
Binnen-I describes writing the letter I of ‘-in(nen)’ in capital, as to indicate the change. This is not the best solution, as some of them might confuse I with a lowercase L.
Liebe MitarbeiterInnen, […]
Colon
This is my favourite way to specify gender-neutral nouns in German. It takes less space than an asterisk or an underscore would, is placed in lowercase and is pretty to look at.
Liebe Mitarbeiter:innen, [...]