Several players were called up from the minor leagues to join the professional team.

were 是 be 动词的过去式复数形式，表示被动语态中的助动词。 called 是动词 call 的过去分词形式，call 本意是"呼叫"，在这里表示"征召"的意思。 up 是副词，表示"向上"或"从较低级别到更高级别"的移动方向。 were called up 是一个被动语态的短语动词，表示"被征召上调"的意思，特指从低级别联赛选拔球员进入职业球队。