Group Task Advice - Assessment Centres

I wanted to find out how I can be more assertive at assessment centres.

It feels like out of my experiences, there's always at least one or two people in my group tasks who are so overbearing and take over the whole group.

This obviously isn't great as you get observed and "marked" on things like contribution.

So, how can more calm and reserved personalities try and take charge and get a word in without being rude?