What to do if one of our supervisors has offered an incentive trip to a certain group of employees but not all?

Except as  per the below, no law requires employers to treat all employees (even employees doing the same job) equally or fairly. The only exception is such differential treatment may not be based on a specifically protected category, of which the main ones are race, religion, sex, age over 40, or disability. If there is no illegal discrimination going on (and no union or individual employment contracts are being violated), then what  you write about should be legal.

An employer can pick and choose who they want to offer bonuses or incentive packages to.  They are not required to give everyone the exact same thing-- even if they perform the same job function.  However, when an employer engages in disparate treatment, they do run the risk of being accused of discrimination.  An employee would have a potential discrimination suit if they could show they were treated differently than other employees in the same capacity and that the reason for the different treatment was because they were a member of a protected class.  Protected classes usually include minorities, women, persons with disabilities, and seniors.  If the group that is being excluded from the incentive trip happens to be a group that is made up predominantly of a protected class, then the group could file a complaint with the EEOC or the Texas Human Rights Commission and allege discrimination or hostile work environment based on the conduct of the employer.