Both are right, but have different meanings. Sentence 1 can be seen as a heading so it is fine by itself. Sentence 2 can be seen as incomplete, but is nevertheless perfectly clear. Here, assist means to help. 1) assisting = the action of helping UI designers in the development of the website 2) assisted = somebody helped UI designers in the development of the website. Hope this helps.

前者是正在进行的动作，后者是过去的动作，也就是现在进行时和过去时的区别。 现在进行时标明正在进行。I am assisting...我正在协助.... 过去式表示过去某一时间的动作。I assisted.....我协助了....一般都会有一个过去的时间状语，例如last week, yesterday 等等。

Hi Luna. Firstly, you have to know that English is language of context. When you say "assisting", the process of assistance (noun form of assisting) is still in motion. When the process of assisting totally stops, it becomes assisted. As the tense of your action changes and shifts to past tense.