The ability to fuse a crew is based on the maximum rarity/star rating that crew can achieve.
The maximum rarity/star rating can be seen at the top of the character, next to its name.
A crew of grey rarity/1 star rating cannot be fused, as it is already at its maximum rarity.
This is also indicated at the bottom of the character.
Characters of a higher rarity can be fused with the same characters to gain a higher star rating, until they reach their maximum rarity.
To be able to fuse, the 2 characters must also be available and not busy.