Force the Groups loop query to get groups having a specific tag

bp-custom.php

<?php

function bp_groups_taxo_loop() {
	$bp = buddypress();

	$reset_term = 0;
	$reset_tax_query = array();

	if ( ! empty( $bp->groups->tag->term ) ) {
		$reset_term = $bp->groups->tag->term;
	}

	if ( ! empty( $bp->groups->tag->tax_query ) ) {
		$reset_tax_query = $bp->groups->tag->tax_query;
	}

	// Set the term
	$bp->groups->tag->term = BP_Groups_Terms::get_term_by( 'slug', 'thetagslug' );
	
	// The Groups loop.
	bp_get_template_part( 'groups/groups-loop' );

	$bp->groups->tag->term      = $reset_term;
	$bp->groups->tag->tax_query = $reset_tax_query;
}