Neutral hydrogen (HI) is a key ingredient in galaxy formation and an excellent tracer of galaxy evolution and the environmental drivers thereof. HI volume density varies significantly with environment, where denser environments (i.e. galaxy groups and clusters) are comparatively HI deficient, but it is not well understood how HI volume density varies over cosmic time. The investigation of the change in HI volume density with these variables, assists in constraining galaxy evolution models. Due to sensitivity limits, HI has not been studied in detail beyond z ~ 0.2, except for objects at the extreme high-end of the HI mass function. With the exceptional sensitivity of MeerKAT, we are now able to explore HI at higher redshifts (z < 0.5 with L-band receivers), and detect a large number of galaxies in a single observation ( > 10), due to MeerKAT’s large field-of-view. This project serves as an exploratory HI study with the new MeerKAT instrument, of a low-redshift, HI-rich galaxy group (z = 0.044), three intermediate-redshift galaxy clusters (0.3 < z < 0.4), and the utilisation of gravitational lensing to search for magnified HI emission behind these clusters to z < 0.5. We present the discovery of a galaxy group in the XMMLSS field, containing 11 HI galaxies, as part of the MIGHTEE Early Science observations. This shows the success of blind HI searches for the discovery of HI groups, and the capacity for the dynamical and morphological studies of resolved HI in groups with MeerKAT. The MeerKAT Galaxy Cluster Legacy Survey sample includes three of the Hubble Frontier Field (HFF) clusters, Abell 2744, Abell S1063 and Abell 370. We perform blind HI emission searches, and HI spectral stacking, a statistical technique used to amplify the signal of low S/N spectra, on these clusters. We present a 7 sigma stacked detection for blue galaxies in Abell S1063, and calculate HI mass detection limits for Abell 2744 and Abell 370. In the blind and targeted searches for HI behind the HFF clusters, we detect no evidence of highly magnified HI emission within the limits of the MeerKAT L-band. The low thermal noise levels achieved in this work, despite short integration times, highlights the enormous potential of future MeerKAT HI observations of dense environments and the intermediate-redshift Universe.