Every new team member (mentee) gets assigned a Mentor - a colleague that has more experience. Some of the responsibilities a mentor has are:
Help with onboarding - New colleague will have a lot of questions about team processes, tools and non-programming related stuff. Mentor is the first person who should help him and lead him to right direction.
Project participation - Mentor should be involved in projects that the mentee gets. He should know the story behind the project and have overview on every project phase. This is important so he can help the mentee with the architecture, planning and tasks. Mentor should feel responsible for colleague's projects.
Task specification - If mentor gives tasks to the colleague, he should take some time in start to define the tasks well. Every task should be explained and it should be clear what needs to be done.
Pull requests - The mentee should send a pull request for every new feature and assign the mentor to review it. Pull requests should be short and for only one feature. Multiple features should not be mixed in one pull request. Mentor should take some time for every pull request that he gets and give advices how the code can be better. Mentor should not write the code instead of colleague but only show him how to write quality stuff. For every advice there should be explanation and some info why it is better that way.
Give feedback - From time to time, mentor should give feedback to the mentee about his progress. He should tell him the things he does well and where he can do better. Feedback is very important for every team member.
Report to team leader and management - Mentor should report a colleague's progress to management periodically. He is the person who should know best how colleague gets along and he should communicate that to team leader and management.