THz technology offers multiple applications in areas such as remote sensing, spectroscopy, biomedical imaging, and ultra-wide bandwidth communications [1]. However, obtaining high-frequency performance at THz frequencies has proven challenging in conventional electronic devices. This difficulty motivated the exploration of unconventional transport mechanisms such as electron plasma waves. Two dimensional electron gases (2DEGs) in semiconductor heterostructures can allow for collective motion of electrons, i.e. plasma waves, whose group velocity is >10X larger than typical electron drift velocities (i.e. vg >108 cm/s) [2-3]. Devices based on electron plasma waves have attracted significant attention during recent years for THz generation, detection and amplification [4]. In this context, efficient coupling of external THz radiation into and out of plasmons in semiconductor heterostructures is essential for the operation of these devices. A conventional approach to excite plasmons in a 2DEG is via a grating gate coupler as illustrated in Fig. 1(a). In a grating gate configuration, adjacent unit-cells interact with each other making this a coupled resonant system. In contrast, via addition of source (S) and drain (D) electrodes, in a HEMT array configuration as depicted in Fig. 1(b), every unit cell becomes effectively independent. In this configuration, the THz to plasmon coupling is enhanced due to a cooperative effect by synchronizing the electron plasma waves in each unit-cell of the array as theoretically discussed by Popov et al [5]. Here we present the first experimental demonstration of enhanced THz coupling to electron plasma wave or plasmon in ultra-thin membrane HEMT arrays via plasmon synchronization. A thin-membrane configuration enables us to remove substrate effects and further enhance the coupling. The proposed approach allows: (i) more efficient excitation of high order plasmonic modes, and (ii) superior overall coupling-even in configurations having less number of devices per unit area-. Our results reveal a simple way to enhance the THz to plasmon coupling and thus improve the performance of electron plasma wave based devices; this effect can be exploited, for example, to improve the response of HEMT THz detectors.