Should i use present participle or infinitive verb

I am trying to construct an sentence but I get a little problem with it. The sentence I want t o make is

When the guests arrived in the centre, they found five greeters standing by the gate welcome [welcoming] them.

I don't know I should use 'welcome' or 'welcoming' here, and why.

It's neither infinitive (welcome) nor present participle (welcoming). Instead, you should use to-infinitive (to welcome).