They always could, but it was rare. Russ and carnifex were squads for years.
Nowdays it's more common, look at the unit entries and you'll see that in tau codex alone, riptides, ghostkneel, stormsurge, hammerhead and skyray can all have up to 3 squad members.
MC. Squad works just like infantry squad, tank squads have slightly different rules, check your rulebook for precise rules.