From ThresholdRPG Wiki

When the difference in guild level between the highest and lowest member of a group exceeds a certain range, the disparity results in everyone in the party being more vulnerable to attack (they will take more damage than usual when wounded). This is because the stronger members have too much of their attention drawn towards protecting the weakest, and the weakest member is doing everything he or she can not to mess up the highly advanced tactics and strategies of the more powerful party members.

The maximum level difference without inducing vulnerability depends upon the guild level of the highest level member of the party.

Highest Guild Level Max Difference
1 - 9 5
10-19 3
20+ 4