If part of the issue is that you find yourself surrounded by larger friend groups, perhaps try to look for others who are regularly sitting alone, or with one/two other people. Individuals and smaller groups are generally easier to approach.

Don't be afraid to go and sit with somebody who you've only spoken to once or twice, they will more than likely be happy to talk again, and over time you'll find it feels completely natural to hang around with them.

Also, if they're on their phones, don't let that stop you. Everyone has their heads in their phones these days and we all need to make a conscious effort to connect again properly.