To join, you must meet certain criteria. An excerpt from the Bylaws states:

In addition to meeting membership eligibility requirements as states in Article IV and Chapter 1 of the Phi Theta Kappa Constitution and Bylaws, each candidate for membership must have completed 12 (twelve) credit hours of course work with a minimum cumulative grade point average of a 3.5 on a 4.0 scale, and adhere to the school conduct code and possess recognized qualities of citizenship. Grades for courses completed at other institutions will not be considered when determining membership eligibility. A cumulative grade point average of 3.25 must be maintained to remain in good standing with the Chapter. An academic probation period of one semester will be granted if a person’s cumulative grade point average falls below a 3.25.
Failure to maintain the required cumulative grade point average will result in the member being removed from good standing as states in the Phi Theta
Kappa Constitution and Bylaws. As stated in Chapter I, Section 3 of the Phi Theta Kappa Constitution and Bylaws, failure to meet good standing
requirements will result in the revocation of membership and all membership privileges.