I don’t see a way to add a user to a channel in the MM app (android, v2.1). A work-around is to tag the user @user in the channel and then you will be prompted to add the user to channel. However, tagging users in private channels prompts to add them to the channel, but users are not added to private channel.
This is a known “issue”, the “add members” feature has not made it into 2.1, the PR is ready, though, so it should be merged into one of the next releases:
The workaround is, as you already said, mentioning users and then click on the “add user to this channel” ephemeral message that appears after you did that.
This feature was available to me up until about two weeks - a month ago or so. I had the option to add people to a channel where I’m an admin from my iPhone. Now, that option is not there.
You maybe updated from the mobile app v1 to the mobile app v2 using your platform’s appstore updates. The new version (v2) is a complete rehaul of v1 and has been recreated from scratch and is still not on the feature level with the previous v1 version in all aspects, but I just got word from the developers that this feature is up for the next version to be released (2.3).