To all teachers at any level, K- Ph.D:

[NYC_SKP]

5. There is no singular solution, it depends upon the group of learners.

Typically, the longer I've spent with a group and the more supportive and well-developed is the group dynamic and classroom culture, the more likely a self-assignment solution will be.

For a short-term or ad-hoc creation of groups, it's often more successful and productive to either randomly assign individuals to groups or, alternatively, strategically assign individuals to reach desired ends.

Naturally, considerations include addressing diversity (of all forms), making sure "weaker" learners are with supportive team mates, etc.

There's never a single simple answer in my experience.