sign up的意思是跟…签订合同; 报名，sign in的意思是通过签字（使）取得入场权; 签到; 画到。

一、sign up

英 [sain ʌp] 美 [saɪn ʌp]

跟…签订合同; 报名;

双语例句

1. He has posted a sign up that says "No Fishing"

他已经张贴了一张“禁止钓鱼”的告示牌。

2. They manage to sign up all the best performers.

他们设法跟所有出色的/卓越的/优异的/杰出的演员签订合同.

3. He was not the only one to sign up.

报名参加的不光是他一个人.

4. Can I sign up for this course in advance?

我能事先申请上这一课程 吗 ?

5. You don't sign up for life, you know.

你用不着参加一辈子的, 你知道罢?

二、sign in

英 [sain in] 美 [saɪn ɪn]

通过签字（使）取得入场权; 签到; 画到;

双语例句

1. All the participants were required to sign in as they entered the hall to attend the meeting.

出席会议的人在进入大厅前均须签到.

2. In the meantime, would you sign in and register your ability?

同时你能签下表格以便登记你的超能力 吗 ?

3. Clerk : Yes, you can't miss it. There's a big sign in front of it.

对, 你不会错过的. 酒家正门挂着一块很大的招牌.

4. It imply the illegal word sign in the data.

数据中含有非法字符.

5. Every student must sign in when he comes back to the college.

每个学生回到学校都必须签到.