There are two issues here.
One is the matter of singular or plural: "each" is singular. So your sentence should be about what an individual contributes, rather than what the contributors do collectively. Fortunately, your chosen verb "may" is the same in singular and plural, so this issue doesn't affect that verb.
Another issue is which pronoun to use to refer to an individual contributor. If you know that all contributors are male, then "he" is fine. However, "he" has the problem that it gives the impression of excluding the female contributors. The solution is to use "they".
So I'd go with "each may contribute what they can". Not because "each" is plural -- it isn't -- but because "they" is inclusive.
theydiscussion is inevitable in questions like these. There's a fair chance that the OP wasn't too familiar with the singulartheyusage. Anyway, if there's a more appropriate tag, feel free to replace mine. – Tushar Raj Mar 05 '18 at 12:28